johnson 25hp

[robertmayton]

i have inherited a mid 70's 25 hp johnson from my brother when he sold his boat.i am currently building a small runabout (11') and plan to use this motor.

the question is.......what can i do to increase the performance and gain a few hp.

i know the basics of engine mechanics and i comprehend the process of building up a performance motor, but where do i get the parts? who will do the machine work (correctly)? how much can i do and still retain some kind of dependability?

everyone says " just buy a bigger motor " but that sucks. i would prefer to change what i have and know that I did that....

also, i understand the importance of engine set-up and prop selection, but i am a big guy and for some reason have it in my head that this old 25 just isnt going to keep my interest for too long in stock form.

thanks for reading and for any info available.

p.s. i couldnt find much info by searching the forum but a guy on "hotboat" siad ya'll may be able to help.

[JohnsonM50]

The guy sent ya to the right place. 1st thing is what year? There are 2 OMC 25s, 1 is a 22 ci. motor - up to 76 then they switched to the 31.8 ci. 35 hp block and detuned it a little.
Not to debunk the 22, a potent motor that can be raced in B mod. [with a race lower unit]. If your heavy tho a 31.8 [big block] might be better for a non race boat. If you went along with those who say a bigger motor is the way, you would get alot more acceleration and a little more speed.
If its a 77 or up you can get a 35 hp carb and re-set the timing from 34 BTDC to 30 [book for 35s]. The 35 has a different crank on some, that wont matter with a light load.. Speaking of light load Ive stripped a big block down to 86 lbs, thats as good as performance work alone. It also has a different tuner but they are very close so that wont matter much. [not interchangeable because of cooling].
Ive written a bunch of backyard mechanic BS here on the 31.8s and got modest results. 36 mph -stock, fishing lower-standard prop to 46 with a Rt hand speed prop on a runnabout and more yet on a hydro before goin to a race lower unit. Mostly tho I say just enjoy it. :cool: