# 1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics



## Johny25

Ok so anyone who owns a 80's Johnson 20/25 has heard of being able to up the HP to a 30/35 well it can be done and I did it with very good results. I also believe this can be done on late 70's and early 90's models but you will have to cross reference the parts differences as I did with mine. 

Ok so I bought a 88' 25hp Johnson several weeks ago for $550 bucks (stole it basically lol) and the first thing I did when I got it home was to find out if I could get more hp out of it. I did a mod on my 87' 9.9 so I was pretty sure I could do it to the 25 also. And sure enough I cross referenced the difference between the 25 and 30/35 in 1988 and the only difference was carburetor and intake manifold. Now I say 30/35 because in 1985 the hp rating changed from the crank to the prop which made the 35hp models into 30hps from 85 on. With an exception in 87 when they made a de-tuned 40 into a 35hp. Anyway so I did the mod and took pics of the differences and decided to share with everyone. The mod cost me a total of about $95 bucks if you include gaskets and carb rebuild kit which is mandatory when buying a used carb. 

Ok here are the differences in the 25 and 30hp (20hp models usually have even a smaller intake manifold but same carb as the 25)
I bought a 1984 35hp carb and intake off of a military 35hp johnson or MARS edition. Which mounted up fine to my 88, got it off of ebay for $70 bucks with no shipping. (guy lived near me so I picked it up)

First pic is the difference in the front of the carbs. The 35 carb is on the left and 25 carb on the right. The throat of the 35 is about 9/16 inch larger in diameter. (if memory serves me correct?)



The second pic is of the back of the carbs which mount to the intake, 35 on the left and 25 on the right. Once again you can see the size difference. The 35 is 1.5 inch diameter and the 25 is 1 inch I believe. Obviously the jets in the 35 are much bigger also. Seem to remember someone saying the 25 carb had a 23mm jet and the 35 had a 32-35mm jet but don't quote me on that.



Next is the intake manifolds. Now I have heard people say they ported out their intake instead of buying the 35hp intake. This can be done but it is impossible from what I see to actually get the volume of the 35hp intake from a 25 or 20hp intake. The 35hp intake body is larger than the 20/25 so you will never get the volume of air flow out of a 20/25 by porting it in my opinion.

Here is a pic of the inside, 25 intake on the left and 35 on the right. See the gasket ring on the 25, the outside of that ring is the actual size of the 35hp intake.


Here is a pic of the 35 in front and 25 behind it to show the size difference again.


And a couple pics showing the difference in width
The 35 at 5 and 5/16 

And the 25 at 5 and 1/16


Here is the intake hole differences.
The 35 at 1.5 inches

And the 25 at just over 1 and 3/8 inches


And here is the 25 carb mounted on the 25 intake


And the 35 carb on the 35 intake is perfectly flush with no flow restriction


And the last difference in the intakes was the primer nipple on the bottom of the intakes, the 35hp intake is much larger than the 25 nipple (had to get a larger fuel line hose for this also)
The 35 nipple

And the 25 nipple which you can barely see in the pic


Now I forgot to take pics of the measurements I made when the intakes were attached to the reed plate but I will tell you this, the 35hp intake when measured through the intake hole to the reed plate is 1/8 of an inch taller than the 25 intake. Meaning that the intake body of the 35 is bigger than the 25 as I mentioned above. So I do not see how you could port the 20 or 25 intake out to match the air flow and volume of the 35 intake.

Ok I never ran the motor on my boat before the mod but I did run it several times in a barrel. And without question the mod made a significant difference in power! Not only could you here and feel the difference but the motor now idles WAY better. And I had done a carb rebuild on the 25 also so I know the 25 carb was fine. It is like the motor was made to have more fuel and air given to it. That is just my opinion and the results I got first hand. Had to do a link and sync to get the advance and carb lined up. I will have her on the water in 2 days and will report how she runs on the boat. 

Here is a pic of the motor, it is an electric start long shaft ( I gave her a new paint job and shiny stainless prop also)


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## bigwave

Very interesting? Will the increase in hp have any effect on the gear reductions to the prop? How much speed do you think you will gain with the increase in HP? Is there and advantage? Just curious. I might just do this to my boat in the future if there would be a gain in speed vs weight of stuff I am adding to the boat. 


Thanks 
Kevin


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## JMichael

Nice, I wish I could do something like this to my old merc 20hp. Did you increase the prop dia or pitch for the added power?


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## Johny25

Ok the 20/25/30/35hps all have the exact same gear cases in most all the years I mentioned so there will be no issues concerning the gear case. It is made to handle this extra 5 or so hp I added. I won't know how much speed I gained because I never ran it on my boat prior to the mod but I have seen plenty of threads where guys have stated GPS speed increases of up 8mph and much better planing ability and better speed while boat is loaded down after this mod. The biggest advantage from what I have researched is the motor runs and idles better (which I will attest to already) and speed increase under a load.

And no I haven't increased the prop dia or pitch yet because I haven't run the boat yet. I will do an RPM check with the 10.5 X 11 pitch stainless I bought. If I am to high RPM then I will go up in pitch accordingly. My boat with just my son, me and all our gear is around 850-900lbs total weight so I won't be running light ever. Add the wife and other son plus their gear and I am going to be over 1000lbs easy a lot of times also. So I may not have any issue with over rev? I will update what I find though 

The reason I made this post is that I read so many threads talking about this mod but nobody had any pics or proof of the differences in the 20/25/30/35hp motors. And some had said just to change the carb and others said just make the intake hole bigger....etc. So I decided to try and clarify and correct misconceptions about this modification to others in the future. It can be done and there is definite gain in HP. This mod would be most valuable on the 20hp since you could gain at least 10hp out of your motor for around $100 bucks


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## bigwave

I really like this thread....I have a newer 25 johnson
and might do this to mine...........If I decide to do this I will do a speed test before and after. I would only do this to offset the weight gain from my mod. I do not care if the speed increases, but I would like the extra hp for planing. Thanks for your post.


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## Johny25

Another thing I forgot to mention was that when I had the 25hp intake on the motor and I took a flashlight and looked through the intake hole with the carburetor off I could see that the intake actually blocked part of the reed plate. This made it more difficult for the reeds to actually open up in my opinion, and caused air flow restrictions. 

Yeah Bigwave if you decide to do the mod let me know your make and model and I would be happy to help cross reference to make sure you can do the mod with your year Johnson. I am not entirely sure you can do it to newer Johnsons but it would be interesting to find out if you could


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## Johny25

Update: I got the motor on the lake today and froze my ass off, 34 degrees out. I am sick ass a dog too but had to run it while I had the break in the weather. Ok so the motor idled and ran like a champ in the driveway while in a barrel. I get it in the water and under a good load and she started bogging out when I tried to open her up. And she died about 50yds off the dock. So off came the engine cover and I started diagnosing. So I found out that she wasn't getting enough fuel and in order to get her to open up and run I had to keep priming the fuel to the intake. So I have some work to do in the garage this weekend. 

But that being said, if I kept pumping the primer until I got her full throttle she ran great at WOT. And the boat is a freaking missile now! I actually had to second guess myself for adding the extra HP to the boat. With just me and my son we had to be doing 30mph easy and maybe closer to 35 which is really fast in a 14'. And when I got the wife and other son in the boat the motor barely hesitated to get on plane. Had to be doing close to 30 with all of us in it. Was like I didn't even put the extra 230lbs in the boat. (wife is not a small lady) I could plane at half to 3/4 throttle no problem. Other than the fuel issue I had, the motor is a freaking beast! It may have been my imagination but I was having a hard time keeping the boat from listing to the starboard side because of the prop torque at full throttle. And I sit on the port side when running the motor so we usually list to port? We weren't out there long because the wife and kids got cold and I wanted to get back and figure out my fuel issue.

All the mph readings were a guess though and probably felt faster because of the 34 degree air on my face lol. Either way, I have more than enough HP now and probably to much actually. I never thought I would say that either.


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## bigwave

Very cool Johny, Fyi my motor is a 2000 25hp Johnson, do you think it is feasible to do that to my engine? It was too dark last night but the serial number plate is fully in-tact with alot more info...its a two stroke short shaft. 

thanks 
Kevin


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## jasper60103

Johny25 said:


> ... All the mph readings were a guess though and probably felt faster because of the 34 degree air on my face lol. Either way, I have more than enough HP now and probably to much actually. I never thought I would say that either.



Sounds like a beast. Nice =D> 

I seem to recall mine was listing port side at one time, and I was sitting starboard side. I've taken my motor off/on many times so it could have been off center.


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## Johny25

Update to my motor not running as well as I wished she had first run. After replacing all my fuel lines I noticed it was drawing fuel and air through the primer line to the intake and spitting fuel threw the line that attaches to the top of the carb while knob is pushed in. See through hoses are awesome  Anyway I am assuming that the O rings are bad inside the primer assembly so I will be removing and rebuilding the manual primer today. This should correct the sputtering and cutting out issue I had the other day.


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## jasper60103

Hey, Johny. Good tip on the see through fuel lines. The leak was on the smaller line? What size did you use? I have a lean condition on my 30 hp evinrude. That's probably something I should check too. 
It's too bad they didn't make a rebuild kit for the primer. Thanks for the update.


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## Johny25

Yes that is the exact line I was talking about. I put the same size line back on, or as close as I could find at least. I had to go to the chainsaw shop to get the see through fuel line. Napa and what not did not carry fuel line that small. It is a yellow color but see through. I also dismantled the manual primer today and I think the check valve it not shutting all the way. Which is allowing air bubbles to go through the line. You can rebuild those primers somewhat by replacing the O rings inside and the spring but the check valves are not replaceable so I am going to order the whole primer assembly and replace it I think.


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## jasper60103

OK, makes sense. I will give it a try. 
FYI. I've found that boats.net seem to have the best pricing on OEM parts.
Even cheaper than my local OMC dealers with shipping charges.
Thanks.


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## DIZZYSPOTS

Thanks for taking the time to post this info...on a parts search now...
also have most of the parts ready for the shifter mod as well

cheers

Mike


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## benjineer

I've got an '03 I want to do the same thing on one day.


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## Johny25

I don't know if you can do it with an 03 by just changing the carb and intake. You would have to do some cross referencing to see what is different between your motor and the 30 on that year.


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## benjineer

Thought it was just the carb, but might be more. When I get ready, I'll look deeper.


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## Johny25

I did a quick cross reference and the intakes are the same. Different carburetor and the cylinder and crankcase assembly has different part numbers #-o I couldn't really tell what the difference was other than the starter bracket but there must be something else I believe. This will be a big road block in doing this conversion if the crankcase is different on the 30 than the 25. But it was just a quick look at them.

This shows the 25 and the 30hp crankcases and the different part numbers.
https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=2003&hp=30&model=J30RSTB&manufacturer=Johnson&section=Cylinder+%26amp%3B+Crankcase


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## lucescoflathead

If I'm reading this correctly, this conversion will work on a late 70's ,early 80's 20 HP Johnson/ Evinrude 2 stroke. Correct?


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## Johny25

Well lucesco I know some of those years you are talking about had different gear cases which could cause a problem when putting 10 to 15 more HP to them. You would have to cross reference to see what the differences are for what year motor you are talking about. I know I was talking to someone with an 80' or 81' and the 20hp of that year has the smaller split gear case than the 35hp. Would hate to see people doing this mod and ripping their gear cases apart so look into it. Or ask and maybe I or another can help you.


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## lucescoflathead

I don't have a 20 Hp yet. What year would you get? I want to try to get one this summer. Thanks Todd


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## Johny25

I own an 88' 25hp that I made into a 30hp


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## lucescoflathead

Thanks for the info. Todd


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## Johny25

Johny25 said:


> Update to my motor not running as well as I wished she had first run. After replacing all my fuel lines I noticed it was drawing fuel and air through the primer line to the intake and spitting fuel threw the line that attaches to the top of the carb while knob is pushed in. See through hoses are awesome  Anyway I am assuming that the O rings are bad inside the primer assembly so I will be removing and rebuilding the manual primer today. This should correct the sputtering and cutting out issue I had the other day.



Ok update to this thread because I hate threads that do not finish. I received and installed my new primer assembly yesterday, got my 88' service manual also yesterday and did a correct link n sync on the motor. I also replaced the thermostat but I don't believe this was part of the issue I had with the motor. The thermostat was more of a precautionary measure. Anyway I took it out on the lake with my 3 yr old today and the motor ran like a swiss watch  The only problem I had was the RPM got to high with the light load and the 10.25X11 pitch SS prop. Good thing I picked up a 10X13 the other day, so I went to shore and put the 10X13 on and no more over rev. I lost a lot of holeshot but gained top end and motor didn't rev to high. I was really surprised at the difference in holeshot from my Stainless 11 pitch to the aluminum 13 pitch! It was very noticeable. I am wondering how much it has to do with the cupping on the nice stainless prop? I am going to run my 11 pitch aluminum now and see what the difference is compared to the 11 pitch stainless. Anyway that is my update


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## jasper60103

Johny,
that's great news! Glad you got it figured out. =D> 

jasper


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## jasper60103

Hey, Johny.

I was wondering if the new primer improved cold starting? 
I always thought these motors were naturally cold blooded, but you may have squashed that theory.
Did your motor sneeze at cold start before replacing the primer? If so, did it sneeze after replacing primer?

jasper


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## Johny25

No the sneezing wasn't really noticeable until the motor got warm so I don't think it helped that much for cold starting. Although I did find out that the plugs that are recommended QL77JC4 do not work as well at cold start as the plugs that came with the motor do. The motor came with NGK buhw-2 gapless plugs in it. I pulled them out and put the recommended champions in when I got the motor home but after I fixed the primer assembly I swapped the plugs back just to see what the difference was. And the NGK's seem to work much better when starting the motor cold. 

To be honest Jasper my motor doesn't seem that cold blooded. Usually even in 20 degree weather she runs pretty damn good even when cold started.


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## jasper60103

Thanks, Johny. 
Yea, my '88 30hp Johnson has either the QL77JC4 or QL82C, not sure which at the moment.

Before cold starting, I normally give the primer a few pulls. Then startup the motor and feather the primer for the first 10 secs or so (instead of just leaving it pulled out during warm up.)

Funny thing is, my 30hp Evinrude behaves the same way.
Its probably unlikely, that I have two motors with worn out primers?
But I guess I should install the clear gas lines (like you did) to make sure fuel is flowing
from the primer during cold start.


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## Johny25

Well if you have a leaky prime like I had just pull the hose going from the prime to the top of the carb while it is running, if there is a lot of fuel spitting out the line while the primer is pushed all the way in then you have a problem. It will spit just a tiny bit or non at all if the primer is good. My bad primer assembly was spitting a lot of fuel into the carb when in the off position which was a big problem getting the motor to run correctly.


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## shawnfish

Johny25 said:


> Well lucesco I know some of those years you are talking about had different gear cases which could cause a problem when putting 10 to 15 more HP to them. You would have to cross reference to see what the differences are for what year motor you are talking about. I know I was talking to someone with an 80' or 81' and the 20hp of that year has the smaller split gear case than the 35hp. Would hate to see people doing this mod and ripping their gear cases apart so look into it. Or ask and maybe I or another can help you.




that was me and i have an 25 not a 20. think the gearcase couldnt handle 10 more ponies?


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## Johny25

Well it may? But I won't put my name on it. My guess is there is a good reason that there are differences in the gear case between the 25 and 35


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## jasper60103

Johny25 said:


> Well if you have a leaky prime like I had just pull the hose going from the prime to the top of the carb while it is running, if there is a lot of fuel spitting out the line while the primer is pushed all the way in then you have a problem. It will spit just a tiny bit or non at all if the primer is good. My bad primer assembly was spitting a lot of fuel into the carb when in the off position which was a big problem getting the motor to run correctly.



Johny, 
thanks for the tip. I'll be sure to check it out.

jasper


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## DIZZYSPOTS

The carb and intake is on...last piece is to find larger diameter fuel line to fit the larger primer nipple...?how did find a suitable plastic "tee" to join the tiny lines to the single larger one? also...the dramatic difference that you illustrated in your post between the 20 hp/30hp intake and carb is nothing compared to the intake on my Johnson 20 the intake had a "restrictor" plate looking casting...the opening is 3/4" with a 1/2" vertical slot at the 6 o'clock position the new carb and intake should be a HUGE improvement...
float test Thurs PM ps the only two props I have two play with are the current 4 blade composite (4x10.5x11) and a stock OMC 4 blade alum (4x10.5x14) results to follow....stay tuned


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## Johny25

I found bigger plastic T's at Napa but I believe in the end I wound up using the same T and cramming a chainsaw fuel line over the nipple on the intake. It took a pair of needle nose to get it on a few times but now I can do it by hand after it stretched a little. \

Yeah I just bought and sold an 85' 20hp with the same restricted style intake. Big difference. Oh and I would go with the 10.5X14 unless you are trying to push a pontoon lol. The 11 will probably cause the motor to over rev with the increase in HP


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## DIZZYSPOTS

got the fuel line at Ace...used same tee..jammed on the lg nipple
test float postponed....my buddy (human ballast for front seat)..got ties up...perhaps Sun stay tuned


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## DIZZYSPOTS

OK only 3 weeks late, but its on the trailer...my lovely wife will act as human ballast and gps watcher...headed to Patagonia Lake in southern AZ for the test float...
G5 sliders also in place.....stay tunes for results


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## DIZZYSPOTS

OK only 3 weeks late, but its on the trailer...my lovely wife will act as human ballast and gps watcher...headed to Patagonia Lake in southern AZ for the test float...
G5 sliders also in place.....stay tuned for results


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## Johny25

will be looking forward to hearing from you this evening : ) Be safe as always


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## DIZZYSPOTS

WOW!...test float err...flight was awesome...to review "20" horse Johnson w/ 14 4 balde prop on a Valco U12...loaded to near gross 55lbs fuel/tank, pax 290 + 170 =515
started right up..slight mixture adjustment...out of the no-wake zone....and crank...up on plane quick....out across the lake, my wife is in front with the gps reading off speeds...she gets to 25 mph (4850rpm) and I roll off the throttle and thats way fast in a tiller-steering 12ft tin boat 

Thanks Johnyrude!!!

a little over 3 weeks and we'll take it across the bay at Gonzaga in Baja...hunter for trigger, yellowtail and grouper for our 3rd annual tin boat regatta


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## Johny25

Awesome  So did you let off the throttle or was there more to be had? 4850rpm is low for that motor, should be hitting 5500 or better after the mod. I am trying to remember what year you have and the exact mods you made? I am surprised that it didn't crank that 14pitch to a higher RPM with the little bit of weight you are pushing although hull design can also be apart of that. And yes 25mph is a good speed for a 12' and anything after that can be scary especially in rough water :lol:


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## DIZZYSPOTS

YES! I let off the throttle...25 in a tiller steered 12ft tinnie is scary fast..a little chop on the lake, not bad...dont know what the other 700 would have got me...not looking cuz i'm happy...gets on plane quickly even part throttle is a nice clip

1987 Johnson 20 hp 30 hp carb and intake swap


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## Johny25

Ok that's right, sorry I have so many motors and mods in my head I cannot remember one from the other any more  So yeah with that 2.15 gear case you should be able to turn that 14 pitch to 5500-5800 and probably hit 28-30mph.....and yes that is scary fast in a 12' tin with tiller steer :lol: You will have to open it up one day in calm water and post results. 

What was your top speed prior to the mod?


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## Jay D

I have a 1988 25hp evinrude, what would it take to boost it to a 35hp evenrude? If you already answered this please forgive me.


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## DIZZYSPOTS

top speed before the mod..hard to get to 18...but had fuel issue" bad"(old) bulb and hose, then bad gas...it got to 20mph..but only once and it took the length of the lake to do it....Now? twist the grip and you're on plane, seconds later 25!
If we get a glassy day in Baja, Ill try to get a real top end for ya


PS 25 days until fresh fish tacos and cold cerveza on the beach...


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## Johny25

Jay D said:


> I have a 1988 25hp evinrude, what would it take to boost it to a 35hp evenrude? If you already answered this please forgive me.



I made this whole thread from the exact year and HP motor you have Jay. Start at the beginning of the thread where I layout and have pics of your exact motor converting into a 30hp. It will never be a 35hp since the 35 was re-badged to a 30hp in 1985 when OMC prop rated the HP on there motors. The old 35's were only cranking out 30hp at the prop hence the reason it was labeled the 30hp after 1984. Read the first page of the thread and let me know if you have any questions


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## Johny25

DIZZYSPOTS said:


> top speed before the mod..hard to get to 18...but had fuel issue" bad"(old) bulb and hose, then bad gas...it got to 20mph..but only once and it took the length of the lake to do it....Now? twist the grip and you're on plane, seconds later 25!
> If we get a glassy day in Baja, Ill try to get a real top end for ya
> 
> 
> PS 25 days until fresh fish tacos and cold cerveza on the beach...



I remember my first day out with mine Dizzy, had an 11pitch Stainless on it. The freaking thing would get up on plane and hit 25mph so fast that it was SCARY. Like a damn rocket. Of course I was way under propped and the motor was turning 6290rpm lol not good. So the 14 stainless was put on which made time to plane much longer but top end was increased about 5 mph. FYI I have had my 14' tin completely out of the water and airborn when running WOT in rough water, that is an awesome feeling. My 6 yr old loves when we wave jump. 

I am glad you are happy Dizzy. Don't worry, you will get used to the extra power after a few trips and start thinking of a 40hp on there just like me :lol:


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## DIZZYSPOTS

actually just thinkin of bigger boat...more room to share fishing with a total of 4 grandkids....


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## Johny25

I hear ya, Ive been trying to find an 18' or better. And bigger boat means I get to play with bigger motor


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## DIZZYSPOTS

Just got back from Gonzaga Bay in Baja...had perfect flat, windless seas...can you say 35 mph in a 12ft tinnie...YEEHA! actually kept up with my buddy's 17 ft bayliner...till I chickened out ")


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## Johny25

35mph in a 12'tin is butt puckering fast! :mrgreen:


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## nick4203

lol my buddy gots a 77' johnson 35 on his 14 foot valco always the fastest tin boat on the lake he keeps talkin he wants a 50 i told him i wont go with him any more haha


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## DIZZYSPOTS

bigger boat gets picked up sat...74 Wellcraft Airslot 185...Evinrude 140 clean...2nd owner covered and in AZ its whole life...so about this 140 hp......


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## Johny25

Depending on the year of the 140 it will decide whether it is a crossflow or looper. Needless to say I am jealous


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## Charger25

Johny25 said:


> 35mph in a 12'tin is butt puckering fast! :mrgreen:




Yeah it is ......... but I bet he had monster grin from ear to ear !! I know I would. :LOL2: :LOL2:


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## DIZZYSPOTS

yeah so did my son in the front of the boat...maybe that was from the wind  


PS think the 'rude 140 is a 87 or 89..168 hrs on it...dont have serial number in front of me...VRO blocked off using premix


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## Johny25

It is a looper then, and if it is an 88' or later it is a big bore looper


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## Plasticmotif

Just joined to say Thanks for this thread!

I've got a 93' Grumman 1465 with a 90' Evinrude 20HP. The boat's a bit heavy and will only do 10 mph going upstream against generators. On the lake, it's fine. I'll probably end up doing this switch to a 30HP. 

Do you have any other notes?


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## Johny25

Well if you want just a little more power you can just bore out the backside of the 20hp carburetor to make it into a 25hp. This is the only difference between the 20 and 25 if memory serves me correct on the 90' models. If you want the extra ten HP then intake and carb will be needed. The 20 had a restricted small hole on the backside of the carb where it attaches to the intake manifold, bore it out to match intake hole size and you got you a 25hp. Don't get any metal into the crank case though so remove carb to do this and clean well.


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## Plasticmotif

dbl post


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## Plasticmotif

Thanks for the fast reply. 

I'll be on the lookout for a 30HP Carb and intake. What years work? Also, If you could point me to where I could find model numbers, I'd very much appreciate it!

I want to be doubly sure before I go buying anything. I've got a CE20CRESB.


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## Natemeins

What a great thread! I'm hoping it will serve me too. I've got an '84 Johnson 30 remote (so 25 at prop?) I'd like to get just a bit more out of. It's on a Grumman 1448 that as a kid my dad and I spent HOURS fishing in, and believe it or not I used to ski, kneeboarding, and tube behind. I know a 30hp is a slightly larger rating for the boat already but after I mod it up (floors, seats, etc) I think I'll need just a little more oomph to pull my kids, and ok maybe myself. :LOL2: Short of doing the research for parts, is it worth it in your opinion? Could I gain just 5 more? Man, what a difference that would make. And just so you know, I've just recently rebuilt the transom to stronger dimensions, so I think the boat will be able to handle the HP. Thanks for your comments.


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## Natemeins

After doing a fair amount of part number cross referencing, it appears the 30 and 35 hp motors in 1984 used identical intakes, reeds, carbs, low speed needles, and high speed orifices. Even identical pistons, rods, and cranks. So how did they manage to get another 5 hp from one to the other? I noticed on boats.net that two different high speed orifices were used in both motors, a 61D and a 63D but it didn't clarify where/when they were used. Part numbers are 0319907 and 0328544 respectively. I can only assume one is slightly larger than the other, is it safe to assume the 63 would be larger, or do they follow drill bit sizes and therefore be the smaller? I'm gonna have to pull mine out and see what's installed.


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## Country Dave

_Hey man very cool thread,

Definitely good info for anyone who want to get a few more ponies out of there OB........................ =D> _


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## jasper60103

[url=https://www.tinboats.net/forum/viewtopic.php?p=333802#p333802 said:


> Natemeins » Yesterday, 3:51 pm[/url]"]After doing a fair amount of part number cross referencing, it appears the 30 and 35 hp motors in 1984 used identical intakes, reeds, carbs, low speed needles, and high speed orifices. Even identical pistons, rods, and cranks. So how did they manage to get another 5 hp from one to the other? *I noticed on boats.net that two different high speed orifices were used in both motors, a 61D and a 63D but it didn't clarify where/when they were used. *Part numbers are 0319907 and 0328544 respectively. I can only assume one is slightly larger than the other, is it safe to assume the 63 would be larger, or do they follow drill bit sizes and therefore be the smaller? I'm gonna have to pull mine out and see what's installed.



61D and 63D orifice plugs are used on the 30 and 35 hp respectively. See marine engine...

https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1984&hp=30&model=J30ECRR&manufacturer=Johnson&section=Carburetor

Good luck.

jasper


----------



## Natemeins

Thanks Jasper. I did some more research on the BRP website as well and from what I can tell the carb, high speed orifice, low speed needle, intake, reeds, and reed stops were slightly different, as they all had different part numbers. As availability of these parts were lost part distributors superceded to the same part that was used in the 87 30hp and sold them to the 84-88 jonnyrudes 20-35hp. So I'm faced with trying the cheap way and try a 63D high speed orifice and hope for the best (I have a 61D in it now) or I'll have to replace the manifold, reeds, reed stops, and gaskets. (They actually sell a "leaf" plate assembly which comes with most of the above parts for a single decent price, but the intake manifold is between $114 and $142 new. May not be worth trying to get 5 hp with all that. It would be nice to have a true 30hp motor though, as opposed to 25hp at the prop.


----------



## RStewart

Thanks for all this info. It will be very helpful. I can't figure something out though. I have a 1999 9.9 Johnson. I was trying to see if I could increase it to a 15 hp motor. My model # is J10REER & I crossed it with a J15REEA, closest model number to mine. I checked carb, intake, crankcase, reed plate, exhaust, pistons, & gear case & both engines have the same part numbers. How does the 15 make more power? What can I do to get more power from the 9.9? It is on a 1968 Rich line 14 ft aluminum semi v & it will not plane out. I do know one of my issues is I have a long shaft motor on a short transom. I would love to get a 25 hp motor & do this mod to it, but for now I'm stuck with the 9.9.


----------



## RStewart

Ok, so after close examination I've learned that I'm a goob. When I looked up offset numbers fit both motors, it gives both carb numbers, fit the 9.9 & the 15. I didn't realize this so the carbs are not the same. Everything else software to be the same so I need to find a carb from a 15 & I'm good. I may just buy a new one. 

Now I would like to know what years would interchange with mine? Or how I can find out what years? Thanks.


----------



## Johny25

My guess would be any 15hp carb from 94' to 2001 but you will have to research it a little more. In 94' they went to the 15.6ci block and in 2002 they started making the 4 strokes I believe with the 2 strokes. But I guess off the top of my head would be that year range


----------



## RStewart

Thanks Johny25.


----------



## bigredxlt

If the 84 johnson that were labeled at 30 are really only putting out 25 at the prop does the same go for the 84 johnsons labeled at 25? are they really only putting out 20?


----------



## Johny25

1984 they did not make a 30hp......was a 35hp. 1985 was first year of the prop rated HP and the 35 became the 30hp. So yes prior to 1985 the engines were not putting out the rated HP at the prop. They were rated at the crank before 85'

I believe they would lose about 10-15% HP from the crank to the prop so if you calculate that out it would be 2.5 to 3.7hp difference on a pre 1985 25hp engines......so maybe 21.5 to 22.5hp at the prop for a pre 85 25hp....hope this helps


----------



## dkonrai

i had a older 72 johnson 20 hp. i stuck a 25 hp carb on it and it flew. it was on a 14 foot duroboat and at wot it was scary fast. sold it to a guy who is still running the motor. i didnt have a gps, but, it planed in seconds but would chine walk at wot.

https://www.youtube.com/watch?v=E5_RQaHvx4Q

i now have a 80 johnson 25, that needed a lot of work. i got it back together and am going to swap out the intake and carb (if i can find a set cheap) . motor seems to have way more power than the older 72 but still has the little split case lower unit. im thinking the lower should hold up as the 50's motors (30hp) had the same style lowers. 

here she is running. i have since installed a lanyard style kill switch (missing in the vid, thats the loose wire) 

https://www.youtube.com/watch?v=sySIsymJeSQ

thanks johny for inspiring us with your great thread,
dino


----------



## Johny25

Nice video of the old 20 screaming around......although my head is now kinked lol. The 72' you had was the small 22ci block which was used on the 20 and 25hp's up until 77' I believe when the bigger 31.8ci block began being used. Those old 22ci motors were very light and had a great power to weight ratio......think that motor you had was likely less than 85lbs as opposed to the 80' you have that weighs 30+ pounds more I would guess


----------



## dkonrai

[url=https://www.tinboats.net/forum/viewtopic.php?p=341349#p341349 said:


> Johny25 » 41 minutes ago[/url]"]Nice video of the old 20 screaming around......although my head is now kinked lol. The 72' you had was the small 22ci block which was used on the 20 and 25hp's up until 77' I believe when the bigger 31.8ci block began being used. Those old 22ci motors were very light and had a great power to weight ratio......think that motor you had was likely less than 85lbs as opposed to the 80' you have that weighs 30+ pounds more I would guess



johny, 
sorry about that kink in your neck. it was so hard to vid and drive at the same time lol. i couldnt wot only in short little spurts. the torque steer and chine walk made my but pucker a bit. yes great power to weight ration. it was a little heavier than my 9.9/15's. trolled nice too (surprising).
i have a 30 hanging off the back of my 16 foot bayrunner, she seems a little under powered  
dino


----------



## ccm

Hey RStewart I think the reason your boat isn't plaining out is because of the long shaft. I have a 1961 Sea King PDUT built by Arkansas Traveler(same as DUT model) I have a 4hp Tohatsu 4stroke tiller model and if I move to the second bench seat the boat will plain out nicely. I have just bought a Stingray Junior Hydrofoil to see if it will plain with me at the back bench. If I were you I might try a jack plait to get the cavitation plate to the same level as the bottom of the hull. And as far as turning my motor into a 6hp it will happen when my warranty expires. (two years to go  ) Good luck to you in converting your motor into a 15hp.


----------



## thill

I have an Evinrude 25 HP engine, E25RERC, a 1994. It pushes the boat great, but the hull was originally rated for up to 60 HP, and it is no where near "scary fast".

Any idea of whether it's possible to upgrade the performance of a 1994 engine? 

Thanks.

-Tony


----------



## Boat2fast

Your engine is a shortshaft? The boat is rated for 60? Either the boat is an early model or the engine has been converted to longshaft. If the boat is a fairly good sized hull, I would start looking for a 40-50hp Johnson Evinrude. This will be a very good upgrade. Take your time and look for a low hours unit made 1982-1988. Late model etecs are good if you can afford 'em.

opinions, 2c


----------



## thill

The boat is a 1957 Crestliner Viking 710 that has been gutted out, the front hood cut back, and a large front deck added. The motor is a 1994 Evinrude 25 short shaft. 


Here is the original brochure- mine is the " 710- decked," the top right picture, pulling the waterskiiers and the third floor plan from the left. The one with the windshield and only two bunks: 

https://retrocrestliner.com/images/1957catalog p9and10.jpg


Here is a picture of my boat currently: 
https://retrocrestliner.com/phpBB3/download/file.php?id=745


I could re-do the transom up to a 20" and use a 40 HP that I already have, but I don't want to add all that extra weight to the back, and the remote controls, if I can help it. That 40 weighs TWICE what this 25 does. I like the additional seaworthiness the lighter rear end gives me.

The 25 pushes the boat well- about 25 MPH- but if I could get a little more power without gaining weight, that would be ideal. Let me know. Thanks.

-TH


----------



## Boat2fast

Nice hull. Nice build-up. Crestliners are well made boats. I had a 1964 welded 14ft rated for 50hp.

Check out some videos. This is a shortshaft tiller engine. This Johnson 40 was a longshaft when I bought it. It has been converted to shortshaft. Shortshaft boats that take big engines are getting scarce.

(Use guest password: twist'er)

https://www.dropshots.com/boat2fast#albums/40hp reconstructed

*********


----------



## thill

That 40 runs nicely! What is your top speed? Looked like you were backing off a bit in the vid.

I'm sure my 40 would be fast, but I tend to go out in heavy seas, and the lighter the motor, the higher the transom floats, and the safer and seaworthy she will be. I've seen the back of a small boat go ker-plunk with too much weight in the back, and it's not a pretty sight.

That's why I'm trying to figure out whether a 30 or 35 carb will do me any good... If not, I think I'm sticking with this engine. It does 23-25 as currently rigged, which I'm happy with. I think I need to go down a prop size, though... not sure yet.

Thanks.

-TH


----------



## Boat2fast

Okay I get the picture. Look for a 28HP SPL late 1980s-97. These engines had the largest carb that the small port block could reasonably use. If you have nice deep pockets, buy a new one if you can find one. Make sure you linkage is correct and the engine is timed to spec. You can cheat 1 or 2 degrees on the timing.

Prop selection really makes a difference. Check with a tach at full throttle-light load. 5500 is max(published) but keep it under 5800. Find the right pitch prop to do this. This will give the best balance of holeshot/top speed/weight carrying. If you carry a spare, make it one size smaller for real heavy loads, pulling water toys etc.

That will achieve most of what you can wring out of a 25.


----------



## thill

Thanks for that info, 2Fast. 

I just made a post asking why my engine bogs down after passing 3/4 throttle and nearing full throttle. I really think it's overpropped. The engine came off a light jon boat, and this is a heavier, decked V-bottom.

It's a non-electric tiller motor, so I don't have a tach, but it sounds like it's not reaching the proper RPM's to my ear.

I'm going to go pull the prop and see what pitch it is... Will post on the other thread to keep it on-topic.

Thanks.

-TH


----------



## jimdandy527

Hello, 

This thread has been super helpful in clarifying some things for me. However, I have not found anything about the effect on a jet motor. We run in fairly shallow water a lot of the time and so we have replaced the prop with a jet. Any thoughts on how the upgrade would affect the performance of the jet, and more specifically do you think the motor would experience any adverse effects? 

Thanks for any insight .


----------



## mphelle

What motor do you have and what is your max rpm with the 5 7/8 impeller? I had an 83 Johnson 35/25 that would only spin at 5200.


----------



## jimdandy527

[url=https://www.tinboats.net/forum/viewtopic.php?p=344697#p344697 said:


> mphelle » 12 Mar 2014, 12:50[/url]"]What motor do you have and what is your max rpm with the 5 7/8 impeller? I had an 83 Johnson 35/25 that would only spin at 5200.



I'm afraid I'm not well prepared, and really not all that savvy at this point... How do I know what the max RPM would be?

I'll get the specifics off the motor tonight. I believe it is a mid '80's model but really can't remember.


----------



## mphelle

A tach is a must have if you're looking for ways to improve the performance of your jet. I've used a digital tiny tach for years with good results, maybe someone here has tried this chinese knockoff.
https://www.ebay.com/itm/Hour-meter-tachometer-jet-ski-boat-sea-doo-polaris-3d-outboard-kawasaki-js-650-/231175992391?pt=Personal_Watercraft_Parts&hash=item35d329b447&vxp=mtr#ht_772wt_758

A jet discussion may be getting a little off topic for this thread, you may want to start your own.


----------



## jimdandy527

Sweet, thanks for the link. I suppose the reason for adding my question to this thread is because I'm looking to do the conversion, but I am unsure of any negative effects a potentially undersized jet may have on the upgraded motor. In reading other posts within this thread it is apparent that with the increased HP a prop adjustment is needed to prevent over rev; unfortunately my limited knowledge led me to wonder if the same would be true of a jet as well or if it will likely have less effect since the mechanics of a jet are so different.


----------



## mphelle

The J/E 20-35 all use the same pump/impeller, there's no worry about over driving that combination.


----------



## Boat2fast

[url=https://www.tinboats.net/forum/viewtopic.php?p=344722#p344722 said:


> mphelle » 12 Mar 2014, 15:58[/url]"]The J/E 20-35 all use the same pump/impeller, there's no worry about over driving that combination.


 A jet pump is like a fixed dynamometer. The more power you have, the more RPM it will spin. It's a water pump...more power in = more water out. That's why a 25 hp will turn it at, say 5200rpm, but a 30hp may turn the exact same pump 5500rpm. 

This is the same idea behind the factory test props. These make 'loading' an engine, at full throttle, safe. Test-props just 'waste' the power. They do not make thrust, forward or backward. The mechanic doesn't have to balance on the transom at 30mph to check the timing. He could run full throttle, in-gear, right at the dock.

The factory manual specifies the right test-prop and the minimum RPM the engine _should_ reach. 1994 OMC factory manual shows the _minimum_ test rpm for the 25hp is 4800rpm, the _minimum_ for the 30hp is 5400rpm. This is using the same test-prop. The stronger the engine, the more rpm it will spin the test prop. By the same reason, stronger engines spin jet pumps faster too.


----------



## iowaevinrude

hey i like your post and pics of your upgrades. a couple ?'s tho, you talked about doing a link and sync, what do you mean by that? also i cant seem to find an intake anywhere unless new, they all look like the 25hp intake on the inside, where it says its a 25 to 25hp intake in the sale heading, same with the carbs. also what is a MARS edition? a motor type or certain model? thanks 
again your post is awesome

thanks
alex


----------



## Johny25

Link and sync is the timing between the carbs and the timing advance as throttle is advanced. The carb and intake can be hard to find used, others have had issues with this also. Just keep searching ebay and other sites like it is all I can say. And the MARS edition was just a military model....not much different than the regular public motors


----------



## shawnfish

[url=https://www.tinboats.net/forum/viewtopic.php?p=351335#p351335 said:



> iowaevinrude » 05 May 2014, 16:29[/url]"]hey i like your post and pics of your upgrades. a couple ?'s tho, you talked about doing a link and sync, what do you mean by that? also i cant seem to find an intake anywhere unless new, they all look like the 25hp intake on the inside, where it says its a 25 to 25hp intake in the sale heading, same with the carbs. also what is a MARS edition? a motor type or certain model? thanks
> again your post is awesome
> 
> thanks
> alex



chances are the intakes are the same and you only need the carb. I have a 35 carb im looking to sell, just the body.


----------



## iowaevinrude

this forum rules. how much do you want for the carb body?


----------



## shawnfish

40 bucks


----------



## iowaevinrude

im sorry i forgot to check for a reply from you on that, i should have left contact information, my name is Alex, feel free to call or text 319-241-3692. thank you for your time


----------



## airbornemike

I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?


----------



## vahunter

airbornemike said:


> I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?


you'll have to cross reference all parts. Its time consuming but thats the only way you'll know for sure. Pull up the parts diagrams for your model number and the 35hp of the same year and style (primer/choke system, tiller, electric/manual start) and first compare the crankcase assembly part numbers. If they match, start comparing every part of the whole outboard.


----------



## airbornemike

Thanks, I'm going to start researching all the parts. It looks like 2001 is the closest year to mine. They didn't offer a 35hp in 2004.


[url=https://www.tinboats.net/forum/viewtopic.php?p=354608#p354608 said:


> vahunter » Today, 17:17[/url]"]
> 
> 
> airbornemike said:
> 
> 
> 
> I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?
> 
> 
> 
> you'll have to cross reference all parts. Its time consuming but thats the only way you'll know for sure. Pull up the parts diagrams for your model number and the 35hp of the same year and style (primer/choke system, tiller, electric/manual start) and first compare the crankcase assembly part numbers. If they match, start comparing every part of the whole outboard.
Click to expand...


----------



## Johny25

They made a 2 cylinder and a 3 cylinder 30hp in 04' and as far as I know neither can be upgraded horsepower wise as they were both completely different motors than the 40hp motors above them. I am speaking about bolt on parts that is


----------



## airbornemike

Ok I was thinking this also, they only made 35hp motors until 2001. Thanks for the info.


[url=https://www.tinboats.net/forum/viewtopic.php?p=354644#p354644 said:


> Johny25 » Yesterday, 23:19[/url]"]They made a 2 cylinder and a 3 cylinder 30hp in 04' and as far as I know neither can be upgraded horsepower wise as they were both completely different motors than the 40hp motors above them. I am speaking about bolt on parts that is


----------



## vahunter

Ive heard the 30/35 intake with more volume doesn't make a noticeable difference. How true that is? Dont know. But if anyone has both intakes laying around plug the carb hole and nipple with duct tape and set it on bench and level the intakes out as close as you can get them. Then fill the intakes with water using a measuring cup keeping track of how much water is in each intake

OR

Level and plug both intakes fill the smaller 20 or 25 intake with water. Carefully lift over 30/35 intake and drain into the 30/35 intake.

THEN 

Measure from the mounting surface to the water. This could give you an idea on how much volume difference there is. Maybe a gasket thickness? Maybe fab up a spacer?

Just thinking out loud...


----------



## vahunter

Anyone have accurate sizes on venturi, manifold side, and mouth diameters? I believe 1.47 for manifold side, 1.48 for mouth and roughly 1.25 for the venturi. Im not sure how close the dimensions are. Im going to try boring out my 25hp carb and making a new throttle plate provided I can find a way to remove my idle and main feed tubes without bending them.


----------



## Johny25

Look at the first page closely and the pic of the 25 intake next to the 30/35 intake. Roughly a 1/4" larger opening all the way around. You can see the gasket line showing the size difference. Not to mention it is a deeper intake. It is a considerable volume increase considering the small size of the intakes to begin with


----------



## foxhunter

i just did this conversion on my 1985 20 hp, the difference in the hole shot was like night and day. propped the moter to 5500 rpm and top speed went from 18-20 mph with stock 20 hp to mid to upper 20"s. the acceleration is awsome.
i have a converted 20 hp inatke to a 1.5" hole if someone needs one.


----------



## vahunter

FYI: 
I did a test with my bigger carb with stock 25 intake and the bigger 30/35 intake. I saw a very very minor change. So IMO im not sure the bigger volume intake matters as long as the hole size matches the carb.


----------



## Sportsman570

I have found this tread to be very informative and thank Johny25 for posting in such detail. My situation is slightly different from most as I'm looking to mod a 1988 Mil. surplus, Johnson 25Hp(AEM25B). It has the same 31.8 Cubic inch power head as ones already discussed. The intake manifold is PN 396788 with the Carb assembly being PN 432168.

As several of you have already done, I'd like to squeeze more out of the outboard and believe that I have understanding of what's required. My intention is to replace existing carburetor with a new or rebuilt Carb. assembly PN 0432021, and to replace the existing intake manifold with PN-0396790. Does this sound correct?

Also, I'm a little confused when it comes to the intake manifolds and their part numbers. I'm having a little trouble locating a used 0396790 intake but have found a used 0391054 for sale. From what I understand the 790 replaced the 054 model. Other than the part numbers are these intakes identical in every way? 

Should I pass and keep looking for used 0396790 or quit being so damn cheap and buy new? I wouldn't think a new manifold would perform and better than a used one that was in good condition but I could be wrong.


----------



## rusty503

You need to check eBay. There is one up there now. Drop the leading 0 and search for 396790.


----------



## Sportsman570

[url=https://www.tinboats.net/forum/viewtopic.php?p=358229#p358229 said:


> rusty503 » 03 Jul 2014, 21:52[/url]"]You need to check eBay. There is one up there now. Drop the leading 0 and search for 396790.




Johny25 are you referring to:

https://www.ebay.com/itm/Johnson-Evinrude-OMC-25hp-Intake-Manifold-P-N-396788-396790-/261353743027?pt=Boat_Parts_Accessories_Gear&hash=item3cd9e5a2b3&vxp=mtr

They listed both 788 and 790 part numbers. But zooming in it is stamped 331454. I'm confused...


----------



## Johny25

I can tell just by looking at the picture on ebay that....that is a 20-25hp intake, not the 30-35 you are looking for. And yes the 0391054 is the same thing as the 0396790. I am sure that the 0391054 is what I had on mine (sold boat and all last week). The 0396790 just replaced the 054 number. If you look at the pic of the back side of the intake compared to the pics on the first page of this thread that I posted you can see it is the smaller intake


----------



## rusty503

Doing another search on eBay I found the 391054 one you want. Do a search for 391054 leaving off the leading 0. One picture even shows it stamped with the correct number.


----------



## Sportsman570

Thanks; I just ordered it. Now I guess I need to start looking for a carb. Are there other part numbers other than 432021 that I should be looking for?


----------



## rusty503

Quick search shows that part number has been replaced with part number 0439389.


----------



## rusty503

Guess what? There is one of them on eBay too.


----------



## Sportsman570

Before I pull the trigger and purchase, I have a few questions. The carb. looks like it has a bracket for an electric choke. My current engine does not have a choke, would this pose any issues (especially starting)? Also, in the photo there does not appear to an adjustment knob for the slow speed needle valve. Do you just use a screwdriver or could this be an issue? One final question, the carb on ebay has a Throttle control cam beside it. Will the carb swap also require a new Cam?


----------



## rusty503

Go to https://www.potomacmarine.com/pages/Oemparts?airbrand=EVI#%2fEvinrude%2fE30TELCCE_1988%2fCARBURETOR%2f27426%2f46344 and check it out there. That is where I got the part number.


----------



## Sportsman570

Johny25, If I'm unable to find a donor carb (as you did), without a choke, do you see an issue with using a 35hp carb such as part number 0439389 and disabling the choke by "locking" it's flapper open or somehow removing it altogether. Remember, my current setup uses a primer vice choke.


----------



## spcamno

Hi Johny25,

Thanks for making this very informative post!

I have some questions for you may be you can help me out.

I have a 1990 Evinrude 20hp (E20CRESB) and when I compare the parts diagram it does show a different carb to a 35hp with same model year and what exactly do I need to get in order to increase the output?

Should I get a carb and manifold from a donor 35hp or open up a restrictor plate mentioned in another thread?

Also, is the 40hp share the same head, block etc and what is the maximum possible I can get this thing to?

Thank you in advance!


----------



## Johny25

Do not think they made a 35hp in 1990.....and if they did it was likely a restricted 40-50hp. No the 40 and 50 had a larger and different block entirely...44 or 45ci I believe


----------



## spcamno

[url=https://www.tinboats.net/forum/viewtopic.php?p=360473#p360473 said:


> Johny25 » 22 Jul 2014, 12:01[/url]"]Do not think they made a 35hp in 1990.....and if they did it was likely a restricted 40-50hp. No the 40 and 50 had a larger and different block entirely...44 or 45ci I believe



Thanks Johny25 for your reply.

After some research on the internet my motor is a 1989 not 90 and found info on iboats someone did a mod on a same model 89 by just open up the restriction plate.

Can you tell me if it this the only thing difference between the 20 and 25?

I remember read something about the cam on the throttle or flywheel to limit the rpm and the carb and manifold is bigger, is that true or only apply to the 30 or 35?

Thanks again for your help.


----------



## Johny25

the only difference on between the 20 and 25 in that year is the restricted intake hole from everything I have seen on the motors that I have had


----------



## erikhill

Hey Johny25, What is the part number for the 35hp carb you used? I just installed the larger intake manifold on my 20hp with the stock carb and noticed a 3mph difference on the water. I still want to put on the larger carb, but I'm having a hard time locating one.

Thanks,
-Erik


----------



## spcamno

[url=https://www.tinboats.net/forum/viewtopic.php?p=360880#p360880 said:


> Johny25 » 24 Jul 2014, 23:20[/url]"]the only difference on between the 20 and 25 in that year is the restricted intake hole from everything I have seen on the motors that I have had



Thanks for your prompt post!

I totally missed this reply until erikhill's post.

It must have gone to my junk mail folder.


Thanks again will report back after I open up mine hopefully next week or so.

Cheers


----------



## alfaguy

I am upgrading my 1988 25hp to 30 and bought an early carb with a choke plate. I am wondering if I can make a hybrid carb with the bottom from the 25 and top from the 30? I would have to insert a primer fitting in the old carb behind the throttle plate. 

Probably safer to use the choke system.

Dave


----------



## vahunter

I think the primer fitting on manifold side is pressed. I don't believe its supposed to be removable could be wrong. By "bottom" do you mean the bowl of the carb? I changed the bowl on my carb and honestly I think youd be better off swapping the plug for the hose nipple and of coarse the high-speed jet/orifice. You will need a special screwdriver to change the hi speed jet though. I made one by grinding the diameter down on a flathead screwdriver so it would fit the screwdriver slot well without damaging the threads in the bowl.


----------



## alfaguy

I received a suggestion to drill my early carb on another post and press in the primer nozzle tube. This sounds like a good idea if I can do it successfully. Has anyone else tried this??


----------



## alfaguy

I installed my 30/35 hp choke type carb on my 25 and she cranked right up. I was surprised to find my intake manifold has the larger hole to match the bigger carb, so the manifold I bought is not needed. 

I am headed to the river tomorrow for a long weekend of hunting and fishing. A great test and fine tuning opportunity,

Thanks to the contributors of this post who offered encouragement - You can dooo it.


----------



## jasper60103

alfaguy said:


> I installed my 30/35 hp choke type carb on my 25 and she cranked right up. I was surprised to find my intake manifold has the larger hole to match the bigger carb, so the manifold I bought is not needed.
> 
> I am headed to the river tomorrow for a long weekend of hunting and fishing. A great test and fine tuning opportunity,
> 
> Thanks to the contributors of this post who offered encouragement - You can dooo it.



Great. Keep us posted.


----------



## shawnfish

Johny25 said:


> Nice video of the old 20 screaming around......although my head is now kinked lol. The 72' you had was the small 22ci block which was used on the 20 and 25hp's up until 77' I believe when the bigger 31.8ci block began being used. Those old 22ci motors were very light and had a great power to weight ratio......think that motor you had was likely less than 85lbs as opposed to the 80' you have that weighs 30+ pounds more I would guess




j25rcs 104lbs
j25rlcs 107lbs
j25tecs 113lbs
j25telcs 116lbs


----------



## shawnfish

dkonrai said:


> i had a older 72 johnson 20 hp. i stuck a 25 hp carb on it and it flew. it was on a 14 foot duroboat and at wot it was scary fast. sold it to a guy who is still running the motor. i didnt have a gps, but, it planed in seconds but would chine walk at wot.
> 
> https://www.youtube.com/watch?v=E5_RQaHvx4Q
> 
> i now have a 80 johnson 25, that needed a lot of work. i got it back together and am going to swap out the intake and carb (if i can find a set cheap) . motor seems to have way more power than the older 72 but still has the little split case lower unit. im thinking the lower should hold up as the 50's motors (30hp) had the same style lowers.
> 
> here she is running. i have since installed a lanyard style kill switch (missing in the vid, thats the loose wire)
> 
> https://www.youtube.com/watch?v=sySIsymJeSQ
> 
> thanks johny for inspiring us with your great thread,
> dino




1980 25's and 35's have same intakes but different carbs so only carb needs replaced.. I have a 1980 25 with 35 carb on it...


----------



## shawnfish

shawnfish said:


> dkonrai said:
> 
> 
> 
> i had a older 72 johnson 20 hp. i stuck a 25 hp carb on it and it flew. it was on a 14 foot duroboat and at wot it was scary fast. sold it to a guy who is still running the motor. i didnt have a gps, but, it planed in seconds but would chine walk at wot.
> 
> https://www.youtube.com/watch?v=E5_RQaHvx4Q
> 
> i now have a 80 johnson 25, that needed a lot of work. i got it back together and am going to swap out the intake and carb (if i can find a set cheap) . motor seems to have way more power than the older 72 but still has the little split case lower unit. im thinking the lower should hold up as the 50's motors (30hp) had the same style lowers.
> 
> here she is running. i have since installed a lanyard style kill switch (missing in the vid, thats the loose wire)
> 
> https://www.youtube.com/watch?v=sySIsymJeSQ
> 
> thanks johny for inspiring us with your great thread,
> dino
> 
> 
> 
> 
> 
> 1980 25's and 35's have same intakes but different carbs so only carb needs replaced.. I have a 1980 25 with 35 carb on it...
Click to expand...

and your question about the lower holding up... yes, yes it will. the 50's thru 80's johnsons were built very very tough and the 25 lowers will handle the xtra hp, mine has with no problems whatsoever...no metal flakes in the lower oil when I have changed/checked mine....


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## shawnfish

johnny25 gave me the idea about going from 25-35hp and he helped me out a lot with my questions etc.... so im gonna make a PDF file with all the part numbers and other information that is relevant to these conversions for motors from mid 70's through mid 90's..... I will post on here when im done and will send it to anyone who wants it... not that johnny25 is being bothered(I think he likes talking about it) but this way people can have a cross reference chart.....


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## alfaguy

Took my 25/30 to the river and it ran really well except for a severe misfire at 2200 rpm. It is fine at 2000 and 2500 but totally breaks up at 2200. It has a new power pack, coils and plugs (but not wires). 

Any suggestions on how to cure it.

Also I could not get above 4500 rpm with a 10-13 prop. I may need more tuning.


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## Siguz

alfaguy said:


> Took my 25/30 to the river and it ran really well except for a severe misfire at 2200 rpm. It is fine at 2000 and 2500 but totally breaks up at 2200. It has a new power pack, coils and plugs (but not wires).
> 
> Any suggestions on how to cure it.
> 
> Also I could not get above 4500 rpm with a 10-13 prop. I may need more tuning.


Sounds to me like it wants more fuel.


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## alfaguy

I think it needs fuel during the transition from low to high speed. I will flush the transfer holes again to see if something got in there after I rebuilt the carb. The main jet is 0.067 so it should be getting plenty of fuel at high speed. 
I am having trouble with the Faria tach above 3500 rpm so I am not sure about my max rpm it is turning.


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## Siguz

I had a similar issue after I did my mod. All I ended up doing is rebuilding the fuel pump and flushing lines. Everything was fine after that. I don't know if that was the cause but it only took 15min.


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## Edstrossner

Great info!

But I am looking to go farther in the 25 to 30 hp mod.
I recently bought a 16 foot Lowe with a 2000 25 hp Johnson, 2 cylinder cross flow.
The stock carb has a .875 Venturi with a 1.250 throttle bore. Wot is 5500rpm at 24mph gps.
I had an old 70 hp carb with a 1.187 Venturi and a 1.375 throttle bore. It bolted up to the intake although the linkage was slightly out of line. The 70 hp carb has a longer throttle shaft. The motor fired up easily with the big carb and idled a bit better than the stock carb. At wot there was only a slight increase, 5800rpm at 25 mph gps. It appears that the motor cannot take advantage of the bigger carb.
I have modified many larger omc crossflows in the past with good success but I need some measurements from a 30hp block If anyone has one. 
I seen one post than indicated that the 30 hp ports are oval. How many ports on the intake side? How many on the exhaust side? What is the deck to the top of the ports measurement? What is the size of the passage way from the crankcase to the transfer ports?
Any info would be greatly appreciated!


----------



## Siguz

Edstrossner said:


> Great info!
> 
> But I am looking to go farther in the 25 to 30 hp mod.
> I recently bought a 16 foot Lowe with a 2000 25 hp Johnson, 2 cylinder cross flow.
> The stock carb has a .875 Venturi with a 1.250 throttle bore. Wot is 5500rpm at 24mph gps.
> I had an old 70 hp carb with a 1.187 Venturi and a 1.375 throttle bore. It bolted up to the intake although the linkage was slightly out of line. The 70 hp carb has a longer throttle shaft. The motor fired up easily with the big carb and idled a bit better than the stock carb. At wot there was only a slight increase, 5800rpm at 25 mph gps. It appears that the motor cannot take advantage of the bigger carb.
> I have modified many larger omc crossflows in the past with good success but I need some measurements from a 30hp block If anyone has one.
> I seen one post than indicated that the 30 hp ports are oval. How many ports on the intake side? How many on the exhaust side? What is the deck to the top of the ports measurement? What is the size of the passage way from the crankcase to the transfer ports?
> Any info would be greatly appreciated!



The best info I can offer is the parts catalogue from Evinrude.
I have been using this to compare parts and fitment.

https://shop2.evinrude.com/Index.aspx?s1=skj55hhutt3n3m40hn0qm2jom5&catalog_id=5&siteid=1


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## Edstrossner

The parts catalogue is a good start but I'll be doing some machine work to this block and it helps to know what OMC did in their port placement to get 30hp. 
I'll be squaring the ports with a 1/4" end mill and possibly opening up the passage way from the crankcase to the transfer ports. It's easy work with a Bridgeport mill. 
I also had a friend of mine point me towards the Chrysler 150hp crossflow. They put in a boost port by drilling a large hole in the piston and the transfer port passage. Interesting stuff.


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## Lil' Blue Rude

I think I'd use at least a 3/8 end mill. Using a 1/4 end mill will give you small radius at the corner of the ports and I think that would be pretty hard on the rings. Seems like some of the inline 6 mercs use a boost port. Haven't heard of many good results from porting on these motors. People talked like they made a little more power, a lot more noise and they drank a lot more gas. I'm sure that was porting beyond 30hp specs though. You might look at scream and fly for what they call the "nine pages" it's mods for the om v6 crossflow motors and I know some people have used the same basics steps on the omc v4's.


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## Edstrossner

I think it was a 3/8" end mill that I used on the V-4's. It was a long reach to square off the exhaust ports and they did not make an end mill long enough in 1/4". 
I do not want to change the port timing on my 25hp. If I did change it, It would be to match up to the 30hp specs. It seems that the 25hp block cannot take advantage of the larger carbs. There must be some flow restrictions that can be opened up. 
Has anyone checked compression on these motors? What is good compression on these? The V-4's and 3 cylinders that I have built would be in the 140 to 150 psi range. After porting I will rework the head to bring the compression up.


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## Edstrossner

We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes!
Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane.
So, the stock head compression is around 100 psi. After my mod, it's at 145 psi.
Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps.
Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph.
I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space.
I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor.


----------



## thill

Edstrossner said:


> We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes!
> Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane.
> So, the stock head compression is around 100 psi. After my mod, it's at 145 psi.
> Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps.
> Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph.
> I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space.
> I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor.



You might want to go up a size on the main jet to compensate and take advantage of the extra air you are giving it. Also, to avoid a lean condition.

When I ran my 1994 Evinrude 25 with the hood off, she gained a lot of pep, and I gained a couple of MPH at WOT. But when I slowed down, she didn't want to idle. I wondered about running lean, so I popped the choke a couple of times, and that kept her going. After a while she would idle without issue. When I ran at WOT again, AGAIN, she wouldn't idle afterward without hitting the enrichener. But after a while, she would idle perfectly.

I believe that without the cowl, I was running lean at WOT, which "dried out" the crankcase. Pushing the key fixed it until the fuel distributed back into the crankcase. Obviously, running lean is NOT something I want to do for long!

So I had a choice: Open up some more air inlets and go up a jet size, or leave the cowl on. I left it alone. 

But if you make more air intake and go up a jet size, I bet you will have a beast on your hands!

-TH


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## Edstrossner

Thanks for the info!

I did check the plugs when I came in. Since I started using the 70hp carb I have been increasing the main jet size. It was at 67 but it appears to be lean still so I just when up to 70. 
I also cut a hole in the hood in the area behind the grab handle, then I shimmed up the grab handle a bit to act as a scoop. It's not a huge opening but as the boat speed picks up, more air will be forced under the hood. I did this to a 70hp a couple of years ago. It works great.


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## thill

Oh, that sounds good.

I need to stop reading this thread. You've got me wondering what would happen if I go up a jet size or two and increase the air intake. 

HMMMMMMM..... 

I'm gonna need a steeper prop. :LOL2: :LOL2: :LOL2: 

-TH


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## Edstrossner

Has anyone tried using different cylinder heads for the 25hp? 
There are 4 different part numbers. 
1980-81---324319
1982-------327672
1983-89---327671
1990-------335655
The 1982 head is supposed to be the best.
1990 and up is the low compression head.
But, how good are the other 2 heads?


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## vahunter

I'm curious why omc used a low compression head for the '90+ 25hp outboards. Aren't they practically the same as the '86-89 outboards? I also thought the oem compression were the same. Am I making sense?


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## thill

I know that somewhere before 1990 they increased the HP output of all their motors, to rate them at the prop, instead of at the powerhead.

Because of this, my 1995 is WAY stronger than my 1981, although the compression is lower. It makes almost a 10 MPH difference. The older motor runs excellent, but it's just not as stong at the prop. I've found this to be the same across the board, especially with my 70 HP motors. The older ones run flawlessly, but just don't have the same grunt as the 90's motors.

It depends on how they achieved the higher HP, as to how you proceed. If they increased the bore and stroke, increased the carburetor and intake volume, increased the max RPM, or whatever, it changes how you take advantage of the differences.

Any idea of WHY the 1982 heads are supposed to be the best?

Of course, this is all theory to me, as I have not actually looked at the specs, and I'm not very experienced with making serious modifications to motors. My job is to get them running well, and keep them as close to stock as possible, so my opinion is not worth too much in this discussion.

-TH


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## vahunter

I believe it was somewhere around 85-86 is when they started rating at the prop. That's what I meant.


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## thill

Hey, I notice that you have a very similar motor and setup as I do. 

Curious questions:
Is yours a tiller or remote steer? 
What speeds do you reach with your 25?
Is your boat loaded heavily or light?

On the Lowe, which was bare, I hit 29 MPH
With the Crestliner, loaded with gear and batteries, only 23 MPH. 

When I took the hood of the 25 off, as I mentioned above, the Crestliner was hitting 27, but it was not sounding good screaming like that. I won't do that again, unless I re-jet the carb to a larger size.

-TH


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## Edstrossner

Omc lowered the minimum octane required for all the 2 cylinder cross flow motors down to 67.
They where selling a lot of these motors to areas of the world that had poor gas. 
They may have opened up the ports to keep the hp rating the same.
I have seen pics of the high compression 25/35hp head on boatracingfacts web site. They say that it is from a 1982 model year.


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## vahunter

Edstrossner, can you benefit from skimming a '90s head or is '82 head a different design in general? I thought about skimming mine but I also wanted to keep my motor dependable. I also swapped intake manifolds to the bigger cavity 30hp one and swear I lost a little power than with my 25hp manifold and 28hp carb. 

Thill, mines a tiller. No PT but a manual jackplate. I'm running 6100rpm at 31mph, if I remember right, on light load. Trolling motor, 2 batteries and myself. Medium load (1 passenger and fishing gear) about 29-30mph. Heavy load (popup duck blind, 2 passengers, dog, 5dozen decoys, guns, etc.) I never checked mph but probably 25ish and forever to plane. I run a 10x13 omc SS prop. I could run a 15p prop in normal conditions but this is a good all around prop for me.

Btw in the summer, due to lighter loads (higher rpm) I run 40:1 fuel-oil mix.


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## Lil' Blue Rude

Edstrossner said:


> Omc lowered the minimum octane required for all the 2 cylinder cross flow motors down to 67.
> They where selling a lot of these motors to areas of the world that had poor gas.
> They may have opened up the ports to keep the hp rating the same.
> I have seen pics of the high compression 25/35hp head on boatracingfacts web site. They say that it is from a 1982 model year.


Didn't the head from the 82' on boatracingfacts turn out to be a second effort high compression racing head? I'm pretty sure second effort or seaway made a high compression head for these motors.


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## vahunter

This flyer was shared with me on BRF a while back.


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## Edstrossner

I measured the piston dome at .625 from the deck to the dome at TDC.
The head measures .650. The gasket adds .050 so at TDC the dome to head squish clearance is at .075.
I like to run a squish clearance at .030 to .035. 
So from a stock head you can mill .040 off the head without any additional milling inside the combustion chamber.
These numbers are from my 2000 Johnson 25hp.
I did a lot more work on my head to bring the compression up to 145 psi .

I don't know first hand about the 1982 head. I would love to see one.


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## Lil' Blue Rude

Yeah that's the one I seen on boat racing facts. I'm guessing you had to weld up the one side of the head and shape to match the piston to get compression up that high?


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## Edstrossner

I finally found a1982 35hp head. It looks like the one from the flyer. 
So, I bolted it up to my 2000 25 hp and I was a little disappointed. This motor with the stock head ran at 105psi. The 1982 head ran at 125psi. I had read that guys were getting 150psi with this head. Port timing on these motors must be a bit different . So, I milled the 1982 head and brought the compression up to 145psi and I ran the boat last Wednesday. There was very little difference from the milled stock head verses the milled 1982 head. 
Well that experiment only cost $65, so not much lost, not much gained. 
Next up I have an old friend sending me some 1-1/2" carbs. I'll be trying those and possibly adding some cone spacers to the reed plate. 
Has anyone compared an aluminum prop vs an ss prop?


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## thill

I've been playing with various props on mine, and YES, it can make a substantial difference. I found a lower pitch stainless prop, with a very noticeable cup gave me much better hole shot, AND a couple more MPH at the top end, also. Both of them running right up to 5500 RPM. Very happy with it!

I believe I was running a 13P aluminum, and this is a 12p stainless. Both 3-blade.

Unfortunately, I got the prop used, from an old parts motor, and I cannot find any marks on it to tell me who it's made by, unfortunately. Otherwise, I'd let you know. 

And it's not a polished SS prop, but one that looks like it's been wire-brushed. A polished one might give even better performance? Or is that just for looks? I don't really know, actually. Is the brushing like the dimples on a golf-ball, actually raising performance??? I really don't know, but imagine the high performance bass props would all be brushed, if that were the case.

But now I'm just rambling about props....


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## Edstrossner

Thanks for the info. I new that an ss prop really helps on the larger hp motors but I have not tried one on the smaller motors. I'm running at 5900 rpm with a 4 blade, 14 pitch aluminum and with a few more mods I'll be at the pack limit of 6100 rpm. I know where I can get a 3 blade, 17 pitch ss but before I drop $200 it's good to know that it will make a difference.


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## thill

If buying new, a lot of shops will let you try the prop, and return it if it doesn't work for you. provided that you bring it back in perfect condition.

Places like West Marine will too, but don't tell them you ran the prop, or they may give you a hard time. Just clean it up perfectly before putting it back in the box, IF it doesn't work for you.

-TH


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## vahunter

I firmly agree about running stainless. I lost my rpm info for my prop tests but I was running an aluminum 10.5x11 omc which wasn't in great shape and got about 28mph and switched to 10x13(I think) omc stainless and running 30mph about 6000rpm or something like that. I also have a better hole shot and load handling. I could probably add more cup to jack the motor up and maintain rpm.


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## Edstrossner

I took the intake manifold off the 25 last week with the thought of adding a pair of cone spacers to the reed assembly. After removing the reed plate I noticed that there is a large difference in the crankcase openings. Cylinder #1 has a larger opening than cylinder #2. So, I decided to tear down the power head and rework the block as follows:
Squared the ports with a 5/8" end mill. This did not square the ports that much. I just wanted to get all the ports set at the same height. They were off by .014.
I milled .062 off the exhaust port bottom to match the piston at BDC.
Milled .012 off the block deck
I did a lot of hand grinding in the crankcase area. My goal was to get the crankcase openings at the reed plate to be close to each other.
I also tapered the back of the cylinder wall as it leads up to the transfer ports and knife edged the transfer ports.
The biggest change will be the boost port. I followed the basic idea that Chrysler used in adding a 1" hole to the piston skirt on the intake side and putting a 1" hole into the cylinder wall at the transfer passage way. This should pump more fuel air mix up to the transfer ports.
I still need to match grind the exhaust tuner where it mates up to the block and also to plug the exhaust relief holes in the tuner.
I hope to get the power head re-assembled this week and try it out. 
I'll post my results.


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## Lil' Blue Rude

You might want to get some kind of epoxy and fill any kind of voids in the crank case to raise the crankcase compression back up some from the grinding you've done. Too much volume in the crank case leads to a sluggish motor do to lower intake velocity from lower pressure.


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## Edstrossner

I got the motor back together and put some hours on it to break it in before I ran it at wot. 
The results were disappointing. I did a lot of work on the block and gained nothing.
After the first run at wot I started to look at the exhaust area where it exits the prop. The aluminum props have a smaller area for the exhaust gases to get out as compared to the stainless. So, I drilled a few holes in the gear case to improve the flow. That netted 1 mph increase. When I can afford it I"ll be going to a stainless prop.
I also reworked my 11 pitch aluminum by milling the exhaust area out a bit plus I added a cup to it.
Next, I found a way to remove the rev limiter on the power pack. I went out again and got the the 11 pitch prop to hit 29mph by tilting the motor up. Without the cup, the prop would have trouble holding it's grip at that setting. A stainless prop with a cup should be the best as others have stated.

In review, I have found that this motor does not respond to internal changes like the bigger cross flows do. Perhaps at higher rpms the results may be different. So for those who would like to up the hp on these later model 2 cylinder cross flows my advise would be to get a carb with a 1.375 throttle bore. I tried the 1.500 bore carb but I lost a little speed with it. Mill the the head to get the compression up to the 145 - 150 psi range and open up the air flow getting into the hood. These 3 mods increase the hp very well.
There is a trick to milling the heads that I have not mentioned. You cannot do a straight mill cut to these heads to get the compression up to 150. There is not enough space between the piston dome and the spark plug. The trick is to angle mill the head.


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## Quercus

Awesome thread. thanks to all contributors!


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## unclerick

I know that this post has been thrashed to death. However, I just came across a part off of Fleabay that I had to buy, just because it made no sense as compared to the part numbers and pictures associated with this post. I paid less than $10.00 and nobody bid on it because of the way it was posted. I have done this conversion several times and many thanks to Johnnyrude for his super doccumention of this conversion. But I have also seen some things I thought could add to this subject. 
I bought a manifold (part # 331518). The seller said it came off a 1976 35HP Evinrude. However, it looked from the rear exactly like the 790 in the picture. I can not get this number to come up anywhere. It is exactly like the 0396790 etc in dimensions all around. However, it doesn't have the boss @ the 4 o'clock position with the part number; instead the part# is between the ribs @ the 5 and 7 o'clock positions. It also has the small nipple. It does have the 1.5 bore, the notch around the drain nipple hole and is the 1/8th" deeper plenum @ 5 3/8" plenum width that is nearly flat across the top. The 396154 cam follower (first used in 1986) is attached and appears to have never been removed from the intake. I went back through the ebay completed search and found that this manifold has been listed 3 times without bid. Hoorah for me. My point is, for those looking at this conversion, don't pass up this manifold!!!
The other item I have found is that the carb off a 1979 55hp is exactly like the 30HP in fitment and 1 1/2" bore/venturi size. The linkage fits perfectly (off the top carb) and has the choke. I have not had a non choke 55 or 50 in the parts box yet so I can't swear to the non choke fitment. Also, the idle system is fixed jet, (like the crossflow V-4's). A little dinking around with the idle jet size and the motor idles and transitions like your momma's sewing machine. The 63D main jet seems to be the ticket @ 1600 ' altitude. These carbs are a nickle a hundred. I have seen many "30" hp carbs on EBay, and the majority are not!!! I have seen many 30/35 intakes and MOST are 792's and 25hp models. 
My latest creation is a 1986 20hp CRLCDC restricted engine, (converted to el) w/120psi compression, picked up from the local college for $100.00, with the above combo, choke shaft removed and the holes plugged with a knurled 1/4 aluminum pins pressed into the holes, drilled for an enrichment nipple, a solenoid mount bowl with an enrichment solenoid off a V-4. Looks like it was made for it. On a 14' Mirrocraft tin runabout went from 21.9 to 33.6 mph (off my Humminbird) w/a 012057 Michigan. Gets on full plane in 5.6 seconds. Just a little more grist for the mill.


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## alfaguy

Does anyone change the plastic timing cam when making the 25 to 30 conversion. Another post indicated they are different, even though the parts breakdowns show the same number for both models. I ordered the part quoted and they are indeed different, and it seems the 30 hp cam does not advance as quickly as the 25. I will look for the part number if anyone is interested.


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## thill

Edstrossner,
I've read your posts with interest, as far as opening up the throats to give the motor more lung capacity, and as far as shaving heads, to increase compression.

But what did you do as far as getting more fuel mixture into the increased airflow? Perhaps, it's as simple as going up a main jet size or two?

DUHHH... Just noticed where you went to a 63D main jet. I had only read the earlier post, and not that last report.

-TH


----------



## bamalivin

I have a 1989 Evenrude tracker 20 hp that I am prepping for this upgrade. I know that yours is a 1988 but are the same parts compatible. I have read conflicting information regrading this part interchangeability. I read the thread and understand that I will also potentially be in need of a intake, I just want to make dang sure before I start ordering parts. I did notice a color scheme difference between your 1988 and my 1989. Mine is just like the one in the video that I am linking to this post (except mine is a 20 hp of course). I appreciate any help and advise that can be passed in advance. ~Billy 

https://www.youtube.com/watch?v=xmckQllkBc4&nohtml5=False


----------



## the-outdoorsman

I was led to this thread by way of google so this is my first post. I am not sure if anyone is following it anymore but I hope so. I have a 1990 Evinrude 20 hp (E20CRESB). From reading through a lot of these posts it seems I should be able to do this conversion. I know I can bore out the hole in the intake to make it a 25hp but I want to convert it to the 30hp. How do I go about figuring out the part numbers for the intake and the carb? I think that is the only two things I need to buy. When I look up a 1990 30 hp, there are several model numbers. 

Any help would be much appreciated.


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## nccatfisher

Well if you googled the original question I would have thought you would have tried a search here for your second one, you would have found it has been asked numerous times with usually the same success. You will most likely have to just dig around to you find them quite honestly. There are a couple old threads here with various numbers that will work depending on model.


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## wmk0002

Interesting that this thread got bumped as I was just looking at it last week. Guy locally is selling a set of a 35hp carb and intake for $50 and has two sets. I was thinking about buying both so I could try this out if I ever get the correct model motor to try it on. Then maybe selling the other one on here. Anyone know what the going rate of the carb and intake manifold would be?


----------



## the-outdoorsman

nccatfisher said:


> Well if you googled the original question I would have thought you would have tried a search here for your second one, you would have found it has been asked numerous times with usually the same success. You will most likely have to just dig around to you find them quite honestly. There are a couple old threads here with various numbers that will work depending on model.


Well I did a search before I posted here and again after and I have not found any carb numbers or a thread that would tell me how to figure it out. I am sure it is how I am searching for it with my key words. I did find a member that did his 1990 motor and sent him a PM but his post is from 2014 so probably a long shot.


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## durco

Johny25 said:


> Well lucesco I know some of those years you are talking about had different gear cases which could cause a problem when putting 10 to 15 more HP to them. You would have to cross reference to see what the differences are for what year motor you are talking about. I know I was talking to someone with an 80' or 81' and the 20hp of that year has the smaller split gear case than the 35hp. Would hate to see people doing this mod and ripping their gear cases apart so look into it. Or ask and maybe I or another can help you.


1984 was the last year of split gear case on 20-25.


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## durco

Johny25 said:


> I did a quick cross reference and the intakes are the same. Different carburetor and the cylinder and crankcase assembly has different part numbers #-o I couldn't really tell what the difference was other than the starter bracket but there must be something else I believe. This will be a big road block in doing this conversion if the crankcase is different on the 30 than the 25. But it was just a quick look at them.
> 
> This shows the 25 and the 30hp crankcases and the different part numbers.
> https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=2003&hp=30&model=J30RSTB&manufacturer=Johnson&section=Cylinder+%26amp%3B+Crankcase


The 9.9 to 15 is the only true swap with just the carburetor change. On the 25-30-35 there is a crankcase change, to wit: the 20, 25 and some 30's have "finger" ports on the chamber intake ports. 35's have "bridge" ports. True 35's will have maybe a little rougher idle and a rougher transition (from low rpm carb circuit to high carb circuit) than a jacked up 20, 25 or 30. Not saying you won't gain from the intake and carb swap; you just won't quite get the full "monty" with it.


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## durco

Johny25 said:


> Jay D said:
> 
> 
> 
> I have a 1988 25hp evinrude, what would it take to boost it to a 35hp evenrude? If you already answered this please forgive me.
> 
> 
> 
> 
> I made this whole thread from the exact year and HP motor you have Jay. Start at the beginning of the thread where I layout and have pics of your exact motor converting into a 30hp. It will never be a 35hp since the 35 was re-badged to a 30hp in 1985 when OMC prop rated the HP on there motors. The old 35's were only cranking out 30hp at the prop hence the reason it was labeled the 30hp after 1984. Read the first page of the thread and let me know if you have any questions
Click to expand...

I have a 1984 35 and a 1988 30. You can really feel the difference between the two; the 35 being stronger. When I change motors on the same hull I have to move the tilt pin because the 35 will porpoise whereas the 30 will not. Same pitch prop (13). Did some research in the parts manual. 1984 35 has a different crankcase than a 1985 30. So, when I went to OMC service school in 2000, I asked my instructor what the difference was. He said, "finger ports" vs. "bridge ports". Bridge ports make the true "hot-rod" 35.


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## durco

Natemeins said:


> After doing a fair amount of part number cross referencing, it appears the 30 and 35 hp motors in 1984 used identical intakes, reeds, carbs, low speed needles, and high speed orifices. Even identical pistons, rods, and cranks. So how did they manage to get another 5 hp from one to the other?


By the difference in the shape of the transfer ports in the block. 30 and 35 have different blocks (machining)


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## durco

Johny25 said:


> 1984 they did not make a 30hp......was a 35hp.


Yes, they did. It was the first year of the 30hp. However, they also made the 35 - it was the last year for the 35 in the U.S. (except for military and the later 3-cyl, etc.).


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## 1648grizz

Great thread, I bought a new 25hp 2 stroke Johnson in 2005. I believe this was the last year for the 25hp 2 stroke . On my 1648 Grizzly I would top out at 29 - 30 mph with the 10x13 prop that came with the motor. That was only me in the boat at 225lbs, I think the boat is around 400lbs.
Been looking into the carb swap and it looks like all I need is the 30hp carb off the 2005 30hp , same intake manifold .


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## 1648grizz

Johny25 said:


> Another thing I forgot to mention was that when I had the 25hp intake on the motor and I took a flashlight and looked through the intake hole with the carburetor off I could see that the intake actually blocked part of the reed plate. This made it more difficult for the reeds to actually open up in my opinion, and caused air flow restrictions.
> 
> Yeah Bigwave if you decide to do the mod let me know your make and model and I would be happy to help cross reference to make sure you can do the mod with your year Johnson. I am not entirely sure you can do it to newer Johnsons but it would be interesting to find out if you could



Great thread Johny25, it looks like the only differences between my 2005 J25TESOM and the 2005 J30RSOD is the carb, same intake manifold .


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## RaisedByWolves

Can someone who has the 30/35hp carb take a pic down the throat of carb so I can see the butterfly in the closed position?

I need to see what the holes/cutouts on the butterfly look like to compare to a carb I want to try out.


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## Matt193

RaisedByWolves said:


> Can someone who has the 30/35hp carb take a pic down the throat of carb so I can see the butterfly in the closed position?
> 
> I need to see what the holes/cutouts on the butterfly look like to compare to a carb I want to try out.



Does this work for you?


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## RaisedByWolves

Matt193 said:


> RaisedByWolves said:
> 
> 
> 
> Can someone who has the 30/35hp carb take a pic down the throat of carb so I can see the butterfly in the closed position?
> 
> I need to see what the holes/cutouts on the butterfly look like to compare to a carb I want to try out.
> 
> 
> 
> 
> Does this work for you?
Click to expand...


Yes, perfect.

Thank you.


The carb I bought has a flat cut to the bottom of the butterfly and only a single hole. I was worried that it would cause the engine to idle too high, but looking at yours and my stock carb it doesnt look as bad as I thought.



It only has a 1 3/8" outlet VS the 1.5 I have read about, but the venturi looks to be the same/similar to yours. My stock carb has about a 1" venturi and 1 1/4" outlet so it should work pretty well.


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## Matt193

RaisedByWolves said:


> Matt193 said:
> 
> 
> 
> 
> 
> RaisedByWolves said:
> 
> 
> 
> Can someone who has the 30/35hp carb take a pic down the throat of carb so I can see the butterfly in the closed position?
> 
> I need to see what the holes/cutouts on the butterfly look like to compare to a carb I want to try out.
> 
> 
> 
> 
> Does this work for you?
> 
> Click to expand...
> 
> 
> Yes, perfect.
> 
> Thank you.
> 
> 
> The carb I bought has a flat cut to the bottom of the butterfly and only a single hole. I was worried that it would cause the engine to idle too high, but looking at yours and my stock carb it doesnt look as bad as I thought.
> 
> 
> 
> It only has a 1 3/8" outlet VS the 1.5 I have read about, but the venturi looks to be the same/similar to yours. My stock carb has about a 1" venturi and 1 1/4" outlet so it should work pretty well.
Click to expand...


That looks pretty close to the diameter of my old 25 HP carb. If you're upgrading from a 20, it should help quite a bit, but if you're coming from a 25, it might not be worth it. What's this off of?


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## RaisedByWolves

Its off of a 70.


The venturi is the real bottleneck and this one is easily 2X larger than the stock carb from my 25.











I only have $40 into it and until I find a 1.5" intake to go with the larger 1.5" carb it will do.

This being 1 3/8" matches the intake I have now.


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## Matt193

I read the 70 HP carbs will work, but I don't remember where. That 25 HP carb looks really tiny compared to the one I had on mine. I'm interested in finding out how well it works for you.


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## RaisedByWolves

Yeah it does seem small for the size engine, but other than pics I only have a carb off an 83 parts motor to compare with and those motors were from the crank rated hp days. 

All in all I’m hitting a speed in the ballpark of what this motor should give me set up as is (27 loaded) so I don’t think I have the wrong stock carb.

I have played with height, the prop, added a foil and saw improvement of some type after everything. The prop is the last thing I changed as my old one was trash.

And while I gained 2-3mph loaded(me and wifey) I wound up with the same top speed with just me in the boat and the RPMs also stayed the same.

That tells me I’m running at the limits of the power I can achieve with this carb.


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## RaisedByWolves

durco said:


> Johny25 said:
> 
> 
> 
> 
> 
> Jay D said:
> 
> 
> 
> I have a 1988 25hp evinrude, what would it take to boost it to a 35hp evenrude? If you already answered this please forgive me.
> 
> 
> 
> 
> I made this whole thread from the exact year and HP motor you have Jay. Start at the beginning of the thread where I layout and have pics of your exact motor converting into a 30hp. It will never be a 35hp since the 35 was re-badged to a 30hp in 1985 when OMC prop rated the HP on there motors. The old 35's were only cranking out 30hp at the prop hence the reason it was labeled the 30hp after 1984. Read the first page of the thread and let me know if you have any questions
> 
> Click to expand...
> 
> I have a 1984 35 and a 1988 30. You can really feel the difference between the two; the 35 being stronger. When I change motors on the same hull I have to move the tilt pin because the 35 will porpoise whereas the 30 will not. Same pitch prop (13). Did some research in the parts manual. 1984 35 has a different crankcase than a 1985 30. So, when I went to OMC service school in 2000, I asked my instructor what the difference was. He said, "finger ports" vs. "bridge ports". Bridge ports make the true "hot-rod" 35.
Click to expand...


Spring is here and its time to start modifying boat motors again!

Does anyone know if there is a way to externally determine whether a motor has finger ports or bridge ports?

Any listing of model years that would have the bridge ports?

I just picked up a 74 35hp with a bad lower unit for dirt cheap and surprisingly after only cleaning the points it fired up and ran pretty well. I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:


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## LDUBS

_*"I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:"*_

And, so it begins . . . :LOL2:


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## RaisedByWolves

LDUBS said:


> _*"I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:"*_
> 
> And, so it begins . . . :LOL2:



New/used lower housing should be here tomorrow. \/ 

Now, having dug into the two lower units I have on hand (76 35hp and a 84 25hp)I find that the 35hp has a much higher gear ratio than my 25hp lower I have from a different busted duck motor.

This means I absolutely HAVE to try this motor out to see what speed difference there will be.

The stock prop for the 35 is 11 x 9 which would be a good starting point for a larger boat, but Im going to try it with one of the many 10 x 13 or even my 10 x 15 prop to see what she does. :mrgreen: 

I know the larger diameter prop would be more efficient, but until I find one......


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## dandrews

Just purchased a new to us rig this past week, 1984 Lund s14 dlx with a 1985 Johnson 25HP tiller, first time looking at this motor. I’m no stranger to aluminum boats, involved with hydroplane racing on the side and know enough to be dangerous. So I will beat on this topic some more and provide some more insights. Touching on the last post early 35’s (1980-83 I believe) ran a 1.93:1 gear that’s why you will see 11X9’s and 10’s on those engines. Haven’t seen any mention of total timing advance, not much talk about reed stops and finger reeds and the all important tuner pipe between the differently rated HP variations of the 31.8 block. If anyone is going to go beyond the scope of switching out parts on these engines I strongly suggest you read up on Gordon Jenning 2 Stroke Theory before you start modifying ports, heads, intakes etc. Have a clearly defined goal of what you are trying to achieve with the engine/boat setup. In my case with be looking for more low, mid and top end gains without sacrificing low speed trolling capabilities. Will achieve this through parts swaps, prop/setup and possibly some exhaust side modifications.


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## RaisedByWolves

dandrews said:


> . Haven’t seen any mention of total timing advance, not much talk about reed stops and finger reeds and the all important tuner pipe between the differently rated HP variations of the 31.8 block.



Hi dandrews!

Thanks for chiming in. My above project got stalled, too many irons in the fire. #-o 

Most guys on this and other “tin boat” boards seem to be after bolt on mods vs tuner type mods. Simple easily repeatable stuff that will get you more top speed but still leave time in an outing to do some actual fishing, vs cursing… err… I mean tinkering. Damn wildlife just does not like oily lures.

Timing advance can be tricky for most people as having a test wheel is the safest way to perform this afaik and those can cut into a budget.

Reed stops always seemed like the holy grail to me with good results having been reported. Only problem in doing this is if not done correctly (and what that is I don’t know) it can lead to reed breakage and ingestion.

I’m a toolmaker by trade and have wanted to make a jig for precisely bending the reed stops evenly along their length but again, time…

I believe this to be the key as most of what I have read about people trying this was shade tree mechanic type stuff. My theory on breakage is they wind up with a high spot or hard edge that causes the reed to get work hardened in one spot leading to failure.


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