# 25hp johnson carb upgrade



## heavy-chevy (Feb 23, 2010)

hey guys, havent been on in a while. boats been sitting for a while. ever since i sank it all the electrical stuff has been giving me trouble so i let it sit for a long while. anyhow, shes back in business now and i figured it was time for an upgrade so i hoped the old johnson up a bit. i guess this would make it around a 35 hp...

started by making up a manifold to adapt the flange style carb hook up to a clamp on set up. had my buddy tig it up for me at my welding class. - free











mounted her up with a piece of rubber hose i got from the home depot - $3






up sizing the fuel line was easy, they use a reducer off of the fuel pump for the oe carb. i just removed it and hooked up some 5/16" braided an line, just because i had some laying around.






next to deal with is the throttle, you can see the plastic arm that used to hook to a rod which pushes the arm on the old carb to turn the butterfly. i "modified" an old oil dipstick tube by massaging and cutting the mounting tab and bending it to turn it into the throttle cable... odd but it works.






here is where the adjustment is at, i just drilled a hole in a nut. need to get a shorter set screw, this ones just for mock up.






pretty much all finished up






compared to the stock carb, from 28 to 34mm.







i researched it and a certain year 35hp used the exact same engine as what i have but just had a bigger carb, i think its 88 or 89, i checked almost every part number on the two engines to be sure. so i think this power head can handle more revs reliably. 

I'l tell you how she runs once i get it on the water, still got to jet it and tune the timing accordingly though.


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## crazymanme2 (Feb 24, 2010)

Whats that carb off of?


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## Loggerhead Mike (Feb 24, 2010)

look'n good, nice weldin


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## heavy-chevy (Feb 24, 2010)

its a keihin 34mm pj oval slide carb, came off of my 250r, i put a 38mm pwk on it.


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## Whoopbass (Feb 24, 2010)

I hope it works. I would be worried that your modified throttle cable tube might get out of whack with all the vibration. 
Did you try and find a used 35hp Johnson carb? I've seen them go for as low as $40 on ebay.

I did the conversion on two motors. One was a 1978 Evinrude 25hp with a jet LU. I swapped the carbs before I ever ran the motor so I don't know what kind of difference it made. It ran fine but I sold the outboard since the jet didn't want to push my boat more then 12-15 mph. I put the old carb back on when I sold it.

The second outboard I done the carb swap on is my 1987 Evinrude 25hp. I attempted it last year and it didn't want to run right. It was during the striper run so I put the 25hp carb back on since fishing was more important then performance. 
I had to turn the slow speed needle out about 4 turns to get it to idle when its suppose to be about 1 1/2 turns out. I figured this out at home so I removed the carb since I didn't want to screw up the motor.
According to the BRP website my 87' motor is different then earlier and later models. The block was even different. I think they only made my motor for one year which seems odd. I'm going to do some more checking on this and give it another try.

Good luck with that mod. You might end up with a beast when its all done.


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## heavy-chevy (Feb 24, 2010)

i looked for the carb but it was too rare to find. the problem with buying a carb from a different year is they are completely untunable and the jetting would be slightly off. this way i can dial it in nicely and it should realistically flow much better then the true 35hp carb anyhow. i need to weld i little collar on the top of the carb to keep the tube in place.


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## Kobie (Feb 24, 2010)

I hope it all works out for you. You have inspired me to make my jon go fast as well, too bad you're so far away or else I would race you. I hoping to find a 25 Evinrude cheaply on craigslist and hit the 40mph mark.

Here's the video of me running the boat.
https://www.youtube.com/watch?v=cNe87kDEF1U

Do you have any video of your boat running?


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## Whoopbass (Feb 24, 2010)

At first I didn't notice the pic of the boat you have your motor on.
My GAWD........How fast do you want to go? 
Is the 32" measurement for the width? 
So your going to have 35 hp powering a surf board?
NUTZ

I have read on iboats where guys pulled a 35hp carb from another motor and were able to get it to work without any issues. 
I've done it on 9.9/15 hp OMC motors and its easy. Last year before I decided to buy a 35hp carb one sold on ebay for $40. Shortly there after I decided to buy one and the next carb sold for $180. I found a running 35hp Johnson on an old boat/trailer for $200 so I bought it. I cut the boat up and put it in the trash can and I had it gone in a month. I figure I can sell the trailer and some of the motor and easily get more back then what I paid.

Looks like your pretty innovative so i'm sure you will get that carb to work.
Give an update when its done. If you have a GPS give the before and after results.


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## heavy-chevy (Feb 24, 2010)

i have a gps but its broken, before was 30mph with 2 people and gear. the prop was too shallow before and my tach wont work now so getting the prop right is going to be difficult. im pretty sure i can use my multimeter as a tach somehow by reading the hz or something from a plug wire. il figure it all out im sure. im considering making some pipes for it but i dont want to have to ditch the cowl, I have been reading and experimenting with 2 stroke porting and piping allot lately...


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## Troutman3000 (Jul 7, 2010)

Bump- 

I was considering this same process to get some extra horses out of my motor. All of the parts are the same from what I can tell except for carb. From what I understand you just need to bore out the air intake manifold to get some more air in and thats it.


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## Troutman3000 (Jul 7, 2010)

https://forums.iboats.com/showthread.php?t=274566&highlight=carb+swap&page=3

I see that the lower end is a little different and could potentially wear down quicker over time with a higher load applied to the gears. Im not sure thats enough for me to shy away from the mod. It seems fairly simple even for someone like me.


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