# squeezing more power out of a 25hp merc?



## gotmuddy (Mar 7, 2012)

I am wanting to squeeze more juice out of my 1996 25 merc with a jet pump. I have found out that they made a 30hp two cylinder but the carb setup is completely different than my 25hp. I was thinking about buying a head and having the surface ground down .040" to boost compression. I may even look at making the exhaust manifold more free flowing, and I could make the bore of the carb larger as well.

Side note, can I hook a tach up on this motor?


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## turbotodd (Mar 7, 2012)

I see you are from Arkansas.

In the state of Ar, it is illegal to modify an outboard for the purpose of more horsepower or speed. Just letting you know. If you get caught out on the water by Mr. Greenpants and he finds out, you'll get ticketed (experience speaking). People mod their motors all the time but that doesn't make it legal.

Now then.....getting more power out of it. Shaving the head can help. But then what about controlling detonation? Increasing compression is helpful but you had better know what you're doing. Otherwise you'll be buying a powerhead in no time flat. 

There is a user on another board who knows quite a bit about modding 25's. Might do some searching. arkansas hunting. net. That motor is a single carb correct?

There's usually more speed involved with playing with motor height, setback, hull shape and hull design than there is by increasing horsepower. I had a customer who bought a 1548 WE with a new 40 hp Yamaha a while back and he was disappointed in 32 mph at full speed. Bought it back to me-I installed a CMC tilt/trim, raised the motor up about 2" and took a few props with me to test it out. From 32 to 43 and we didn't increase his HP at all. Just made his available horsepower more useable.

With the jet, it's down on power already. They're a challenge. If you want to increase the power through the jet, you have to also change the impeller to allow the engine to spin up to the right RPM. It's not nearly as easy as a prop drive.


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## JMichael (Mar 7, 2012)

turbotodd said:


> I see you are from Arkansas.
> 
> In the state of Ar, it is illegal to modify an outboard for the purpose of more horsepower or speed. Just letting you know. If you get caught out on the water by Mr. Greenpants and he finds out, you'll get ticketed (experience speaking).



Wow, born, raised, and lived a good portion of my life in AR and never heard that mentioned before. So you're saying that modding is illegal even if it's not being used on HP restricted waters?


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## Lil' Blue Rude (Mar 7, 2012)

More power out of jet. Music to my ears :mrgreen: 
I'm not real sure but I don't thing the smaller mercs have a removable head. I think it's just has a cover over the head for the waater cooling jacket. Like I said not real sure, I've messed with the 9.9/15hp, 40,50,60hp mercs but never the 25hp. If the heads is removable you want to get compression up to 140psi or around that. You should be safe around there on pump gas. There's a product called Aces that is a additive to prevent detonation. Everyone on scream and fly swears by it. Alot of mercs guys that run jets say lightening a fly wheel give alot more get up and go from bottom to mid range. Just gotta find someone to do it and balance it afterwards. Polish the entire exaust system from the port opening to the end of the tuner. Make sure the reeds stops are opened as far as they can be opened. A set of Chris carson reeds would probably help out a little.(Great guy, good product =D> ) Clean all of the casting mark out of the inside of the pump. Bevel the leading edge of the shoe. Getting the motor height just right is good iea to get the most out of it. Check diffrent models motors, you might find a bigger carb that will bolt on, you'll probably have to rejet it afterwards. If you wanted to get real crazy you could port it, but that takes some time and research and a good steady hand or you could have a new boat anchor.
I'm pretty sure you can hook a tiny tach up to it.
As for changing impeller, not really an option. what ever size came in the pump is probably gonna be what your stuck with. It's hard to make more power then the standard size impeller will take and outboadjets doesn't put too small of a impeller in from the factory.


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## dyeguy1212 (Mar 7, 2012)

turbotodd said:


> I had a customer who bought a 1548 WE with a new 40 hp Yamaha a while back and he was disappointed in 32 mph at full speed. Bought it back to me-I installed a CMC tilt/trim, raised the motor up about 2" and took a few props with me to test it out. From 32 to 43 and we didn't increase his HP at all. Just made his available horsepower more useable.



Any more details on the set up? I'd love to squeeze an extra 10 mph out of my set up =P~


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## gotmuddy (Mar 7, 2012)

turbotodd said:


> I see you are from Arkansas.
> 
> In the state of Ar, it is illegal to modify an outboard for the purpose of more horsepower or speed. Just letting you know. If you get caught out on the water by Mr. Greenpants and he finds out, you'll get ticketed (experience speaking). People mod their motors all the time but that doesn't make it legal.
> 
> ...



I am not worried about that in the least. So far in three years I have NEVER seen a game warden in AR or MO. Buffalo, white, 11pt, CC, spring, southfork, lake norfork, NFoW and no game wardens. I will probably see one next time out though. :mrgreen: 

setback is bad for jets and the height is perfect.

semojetman, thanks. The problem with my intake/impeller is its wore out. I can probably lighten my flywheel myself on the lathe.


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## Skidz (Mar 7, 2012)

turbotodd said:


> In the state of Ar, it is illegal to modify an outboard for the purpose of more horsepower or speed. Just letting you know.



Just curious as to where you found this information. Looking through the Annotated Code, I was unable to find any reference to modifying an outboard engine. If you go over the stated capacity of the boat plate, sure, but not pertaining to the outboard alone.

Thanks, 
Skidz


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## gotmuddy (Mar 7, 2012)

Skidz said:


> turbotodd said:
> 
> 
> > In the state of Ar, it is illegal to modify an outboard for the purpose of more horsepower or speed. Just letting you know.
> ...


the only thing I could think of is the buffalo river with a 9.9hp rating. get caught with numbers that dont match and its a huge fine...from what I hear.


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## turbotodd (Mar 8, 2012)

I'm going to have to back up and state what I was "told". A (now retired) wildlife officer explained to me that "any modification that increased power beyond what the original HP rating" is illegal. The boat that we were in when we "found out" about this was rated for 25 HP, and that's what was on it. A 25 Yammie triple that was tweaked a little. We were asked how much HP it was making, we guessed around 40. Wasn't my boat-belonged to a friend, and he was in control of it at the time, so he was verbally warned-and told that if he was caught again, same motor, same boat, there would be fines involved.

Keep in mind this was approx 15 years ago-about the last time I did any duck hunting. I was not born/raised here but forced to move here, and I realized very quickly how competitive duck hunters are here. Too competitive to have fun, IMO.

EPA regs might have some bearing on it too. Not much is enforced here but the more people that are modding their 25's to make 50-60-80 or more HP on 1542 flat bottom duck boats, there might very well be more enforcement in the (near) future. G&F commission knows about the mods and they do check things out given the chance. Especially if one of them is in a bad mood. I got drilled at the Little Red about 3 months ago. Guy checked EVERYTHING. Registration. License. Tackle. Boat rating. Floatation. PFD's. Extinguisher. Everything. By the time he was done, I just got back in the truck and went back home (about 2 hr drive). Ruined the day. He was a jerk but doing his job which is to make boating and water sports safe for everyone. A 1542 going 50 mph is not exactly "safe"-given the hull design and (lack of) weight. Seems that arkansas code has provisions for unsafe operation of vessels. Most other states do and Fed does as well (Coast guard...I lived in a housefull of "coasties" in another state when I was younger). That's a grey area that they can use to their (G&F) advantage....if they wanted to.

One of the (very) few things I've learned about modding outboards....and 2 strokes in general is that there always comes a day when you wished you'd left it stock. Been down that road more times than I can count on both hands and probably toes too. Last bass boat I owned was an 18' skeeter with a modded Vmax 150. Made around 260 HP. Fun but constantly fiddling with it. Either wouldn't idle for poo or it would idle great and crap out on the top end. After the first 2 powerheads, I gave up ....sold it and went back to a little 90 Mariner on a 16 footer. Much more enjoyable. Hit the key, run across the lake and head back after a day's fishing....and no screwing with fouled plugs, holed pistons, etc. Getting there....fast..... is aweful fun, but it's a lot less fun when you can't get back. And it ALWAYS happens when a storm is brewing, the wind is blowing in your face, and lightning is flashing all around. And you're the only boat on the lake. Worst possible time. I believe they call it "Murphy's Law".


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## JMichael (Mar 8, 2012)

turbotodd said:


> I realized very quickly how competitive duck hunters are here. Too competitive to have fun, IMO.



Man you got that part right. I grew up and graduated from HS with a lot of them and they can be absolutely stupid when it comes to duck hunting. I hunt most types of game in AR, but I gave up on ducks after about 10 years of it because it just wasn't worth the hassle for me. 

I don't know whether there is or isn't a law regarding engine mods, I just never heard of it or anyone being questioned about it by the G&F.


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## gotmuddy (Mar 8, 2012)

turbotodd said:


> I'm going to have to back up and state what I was "told". A (now retired) wildlife officer explained to me that "any modification that increased power beyond what the original HP rating" is illegal. The boat that we were in when we "found out" about this was rated for 25 HP, and that's what was on it. A 25 Yammie triple that was tweaked a little. We were asked how much HP it was making, we guessed around 40. Wasn't my boat-belonged to a friend, and he was in control of it at the time, so he was verbally warned-and told that if he was caught again, same motor, same boat, there would be fines involved.
> 
> Keep in mind this was approx 15 years ago-about the last time I did any duck hunting. I was not born/raised here but forced to move here, and I realized very quickly how competitive duck hunters are here. Too competitive to have fun, IMO.
> 
> ...



I'm not wanting to go that insane with it just yet. I really just want to get it back to 25hp at the pump.

**edit** if I can get my 1448 to do 25mph on the 11pt then I will be incredibly happy. 30 and I will be ecstatic.


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## Lil' Blue Rude (Mar 8, 2012)

Don't thinnk you'll be able to get that much more out of it. I've been looking around and from what I can tell your motor is a built up 15hp already.
Curios what speeds are you getting now?


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## gotmuddy (Mar 8, 2012)

Lil' Blue Rude said:


> Don't thinnk you'll be able to get that much more out of it. I've been looking around and from what I can tell your motor is a built up 15hp already.
> Curios what speeds are you getting now?


keep looking. 25hp motors are a completely different everything than a 9.9, and it weighs about 40lb more. I own a 1998 9.9 mariner.

with the 25hp right now I can do 20 by myself and 16 with my 120lb friend in front.


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## Lil' Blue Rude (Mar 8, 2012)

Looking at a wiseco catalog and it says the Mercury 15HP,15 Sea Pro, 15 Marathon uses the same piston as the 25hp and they are listed as the same block#. Here's the link you can look for your self. https://www.wiseco.com/Catalogs/Marine/Mercury.pdf They are definelty the same bore and use the same piston. Or i guess I should say from what wiesco has listed they should be the same.


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## Skidz (Mar 8, 2012)

OP, sorry for hijacking your thread, I will be watching with interest, as I love to find ways to squeeze a bit more juice from engines.

TurboTodd, thanks for the clarification, and I agree with you about duck hunters, and duck hunting in general. I still love it, but it has become so competitive and such a hassle, that I haven't been since the early 90s.

Skidz


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## gotmuddy (Mar 8, 2012)

Lil' Blue Rude said:


> Looking at a wiseco catalog and it says the Mercury 15HP,15 Sea Pro, 15 Marathon uses the same piston as the 25hp and they are listed as the same block#. Here's the link you can look for your self. https://www.wiseco.com/Catalogs/Marine/Mercury.pdf They are definelty the same bore and use the same piston. Or i guess I should say from what wiesco has listed they should be the same.



the bore is the same, the stroke is different. The block is different. The carbs are similar, the lower units are completely different, the shift mechanism is different, the tilt lock is different, and the weight is way off. I am saying it because I own a 1998 mariner 9.9 as well as the 25hp merc.

displacement on the 9.9 is 16ci, and the 25hp is 24.4ci.


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## Lil' Blue Rude (Mar 8, 2012)

Like I said same bore same pistons. I just thought they might have been the same block because the wiseco catalog had both motors with Mercosil Block G202750. Just thought that the 25hp was a built 15hp. I was wrong.  
How did you set the height of your motor? the best way I've figured out is to adjust the motor as high as possible without it cavitating when you try to jump on plane. 1/4" can make a huge diffrence on top end numbers.


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## gotmuddy (Mar 9, 2012)

Lil' Blue Rude said:


> Like I said same bore same pistons. I just thought they might have been the same block because the wiseco catalog had both motors with Mercosil Block G202750. Just thought that the 25hp was a built 15hp. I was wrong.
> How did you set the height of your motor? the best way I've figured out is to adjust the motor as high as possible without it cavitating when you try to jump on plane. 1/4" can make a huge diffrence on top end numbers.



Originally I set it up to be 6" higher than stock, which ended up being a inch too tall so I cut it down an inch and now its perfect. while planed out the front of the shoe isnt kicking any water up and you can see the very front. If I get into a big turn the jet cavitates.


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## Lil' Blue Rude (Mar 11, 2012)

Sounds like the motors about as high as you can get it. Best I can say is polish the pump and keep the impeller sharp.


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## gotmuddy (Mar 11, 2012)

I am going to call Fred tomorrow and order a new intake impeller. I am ordering a tiny tach to make sure I dont blow the motor up.


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## JMichael (Mar 11, 2012)

gotmuddy said:


> I am going to call Fred tomorrow and order a new intake impeller. * I am ordering a tiny tach to make sure I dont blow the motor up*.



I've thought about buying one of those several times but every time I think about the fact that you can't replace the battery in them and they're worthless after it dies, I decide against one. I have seen a clone of the TT then I'm considering now though because you can replace the battery on them.


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## Lil' Blue Rude (Mar 11, 2012)

gotmuddy said:


> I am going to call Fred tomorrow and order a new intake impeller. I am ordering a tiny tach to make sure I dont blow the motor up.


As long as the impeller is the same size that should have come in it your not going to have to worry about pulling to many rpm. It takes ALOT of power to pull a outboard jet past 6000rpm with the stock impeller. If a stock motor does pull past 6000rpm It's setup wrong. Past 6000rpm the impeller start to loose ground or I guess a better way to put it is it can keep up with it's self and it can't pick up enough water so even though your turning more rpm your not gaining any speed.
Any way shouldn't have to worry about blowin the motor up because of pulling to many rpm. But I can understand wanting to be careful with it too.


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## gotmuddy (Mar 11, 2012)

Lil' Blue Rude said:


> gotmuddy said:
> 
> 
> > I am going to call Fred tomorrow and order a new intake impeller. I am ordering a tiny tach to make sure I dont blow the motor up.
> ...



I am going to modify the timing and want to be careful. I am also going to lighten the flywheel.


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## Lil' Blue Rude (Mar 11, 2012)

Be careful with timing because it's easy to get to much and have problems with detonation. I'd take it a degree at a time. That and the flywheel should be good for some more pep out of the hole. Be careful not to take to much off of the flywheel and make sure it's balanced afterward, don't want it coming apart. You might check with Chris Carson Marine to see about a set of reeds for it. That sould help it out with the low end some more.


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## gotmuddy (Mar 11, 2012)

Lil' Blue Rude said:


> Be careful with timing because it's easy to get to much and have problems with detonation. I'd take it a degree at a time. That and the flywheel should be good for some more pep out of the hole. Be careful not to take to much off of the flywheel and make sure it's balanced afterward, don't want it coming apart. You might check with Chris Carson Marine to see about a set of reeds for it. That sould help it out with the low end some more.




I have been talking with a guy that races these motors, he suggests using stock rubber coated reeds.


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## Lil' Blue Rude (Mar 11, 2012)

The reeds cages should be rubber coated not the reeds them shelves. Reeds are cheap and if they don't work you can always but the steel reeds back in.
One thing I've learned since I've been working on hot rodding my motor is what works for one might not work for another so try it yourself that way you know if it works for you. Don't mean that in a smart way either, I've just done things that wasn't suppose to help and they did.


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## gotmuddy (Mar 11, 2012)

Lil' Blue Rude said:


> The reeds cages should be rubber coated not the reeds them shelves. Reeds are cheap and if they don't work you can always but the steel reeds back in.
> One thing I've learned since I've been working on hot rodding my motor is what works for one might not work for another so try it yourself that way you know if it works for you. Don't mean that in a smart way either, I've just done things that wasn't suppose to help and they did.




I haven't decided how far I want to go with this motor. I am one of those poor souls who absolutely cannot leave something alone. I am kicking around the idea of a new jet pump. I think there is a way to get more power to the water with a different design on a pump.


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## Lil' Blue Rude (Mar 11, 2012)

I know what you mean about not being able to leave something alone. As far as I know they don't make any other type of outboard jet pump. What do you have in mind for a new jet?


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## gotmuddy (Mar 12, 2012)

Lil' Blue Rude said:


> I know what you mean about not being able to leave something alone. As far as I know they don't make any other type of outboard jet pump. What do you have in mind for a new jet?



I dont know if anyone has done this but I was thinking of having a small, simple gearbox to turn the power 90 degrees then use a impeller setup just like a current jet. the inlet would rap around the gearbox. My theory is the water won't have to turn which loses your power.


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## Lil' Blue Rude (Mar 12, 2012)

Are you talking about something like an inboard jet? Straight in then straight out the back? Don't know how much it would help out taking the bend out. Be interesting to see though.
Only other jet that I can think of is a durajet but it can't run as shallow as a outboard jet. I don't kno wif they will run much shallower then a prop.
If you want to improve your pumps effeincy you can bevel the leading edge of the liner (wear ring). You don't want to go to far up, just about to the edge of the impeller blades. Not gonna make a big diffrence but if you gotta lathe it would be easy enough to do and enough small things will finally make a noticble diffrence.


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## gotmuddy (Mar 14, 2012)

Lil' Blue Rude said:


> Are you talking about something like an inboard jet? Straight in then straight out the back? Don't know how much it would help out taking the bend out. Be interesting to see though.
> Only other jet that I can think of is a durajet but it can't run as shallow as a outboard jet. I don't kno wif they will run much shallower then a prop.
> If you want to improve your pumps effeincy you can bevel the leading edge of the liner (wear ring). You don't want to go to far up, just about to the edge of the impeller blades. Not gonna make a big diffrence but if you gotta lathe it would be easy enough to do and enough small things will finally make a noticble diffrence.




I will do that when I get a new intake.

A little update: I checked my compression today and it is 115-120psi. I found that mercury offers a .030 piston for a '96 model. Would it be worth my time to bore it? Also, my liner/ impeller are wore enough that with all the shims on top of the impeller I have a 1/8 of a inch clearance or more, that would likely kill my thrust wouldnt it?


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## Lil' Blue Rude (Mar 15, 2012)

You probably won't see any gains from rebuilding the motor since there is only 5psi diffrence. You could do a leak down test. I think as long as it under 10% it's fiine. A new impeller and liner would probably help out some, not sure how much thrust your losing but I'd say it is.


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## gotmuddy (Mar 16, 2012)

yesterday afternoon I put a fresh coat of jb quik weld on the liner and smoothed out the leading edge of the impellor. I noticed the boat planes alot faster.


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