Put 35 hp carb on 1981 25 hp Johnson? Good or bad idea?

[newbie38]

I just got a 1981 25 hp Johnson (model# J25TECIM). I have been told that the powerhead for the 25 and 35 hp are identical, so if I switch to a 35 hp carb I will effectively have a 35 hp engine. Another mechanic told me that since my model is not an exhaust prop, the most pitch I can go on the prop is 12 pitch (which it currently has) and that if I put in the bigger carb I would end up just jacking up the RPMs, giving no additional power and eventually damaging the motor. The motor will be pushing a 15 ft Zodiac inflatable (fairly heavy/inflatable keel), and will usually be carrying 2-4 adults with dive gear. I'm getting conflicting advice from different mechanics who both say they know what they are talking about. Any suggestions? Thanks.

 

[Silvertip]

Re: Put 35 hp carb on 1981 25 hp Johnson? Good or bad idea?

While I'm not intimately famiiar with those engines, your question gets asked a lot about similar comginations and the result is always the same. Except for some of the smaller engines, rarely is bigger carb the only change to create the next size engine. In your case that's a 50% increase in power. That said, I would venture that the intake manifold bore is not the same so you will not get the full benefit of the carb. Although the displacement (CC) of the engine may be the same, I would guess the porting in that engine is not and the exhaust tuner may be different as well. So there is only one conclusion you can draw -- the bigger carb will suck more gas and you will likely not notice a 10 HP difference in power.

 

[newbie38]

Re: Put 35 hp carb on 1981 25 hp Johnson? Good or bad idea?

Thanks Silvertip. Quick question: if I have someone offering to do it cheap, would any horsepower gained be worth it in light of the increase gas consumption? Also, do you have any opinion as to whether it would hurt the engine (the increased rpm on a 12 pitch prop that may already be maxed out)?

 

[F_R]

Re: Put 35 hp carb on 1981 25 hp Johnson? Good or bad idea?

OK, I don't always do this, but I did the homework and it would appear that in your case the 25 and 35 are the same powerheads. So the carb is the difference. And the exhaust housing and lower units.

The 25 is rated @5000rpm, the 35@5500rpm. Approved full throttle operating range is 5000-6000rpm on the 35, but 4500-5500 on the 25.

So, if the powerheads are the same, why the different approved full throttle operating range? My guess is the lower unit. The 25 has all bronze bushings while the 35 has ball and roller bearings. Can the 25 gearcase stand prolonged operation at 6000rpm? I dunno. I do know that my 35 runs @ 6000 with ease.