1981 25hp vs. 35hp specs

Hi, all.
I'm a newbie with a 1981 25hp Johnson.

My OMC service manual covers the 25hp and 35hp models for 1981.
I was reading and trying to understand where the 35hp model
gets the extra hp from?
In comparing the Powerhead specs, seems most of the numbers are the same, e.g. piston displacement is 31.8 cu in. on both motors.

The only difference I saw was the Tank test Wheel Number and Full throttle operating range.

25hp... Wheel Number 388880, 5200rpm
35hp... Wheel Number 386891, 5300rpm

25hp... Full throttle operating range 4500-5500 rpm
35hp... Full throttle operating range 5200-5800 rpm

I don't know what this means.
Can someone enlighten me?

thanks,
jasper

Re: 1981 25hp vs. 35hp specs

I think that if you study the parts lists for the 20 HP through the 35 HP motors between 1976 or so and the late 1980s, you will find that the differences are the intake manifold, the carburetor, and the prop. It seems that the 20s had a restriction cast in the intake manifold reducing the volume of air/fuel that can be taken in. Grind this out with a hole saw or dremel and you get a 25 HP. The 30s and 35s had a larger carburetor. Use one of these on the 25 and you get 30 or 35 HP. I can't find much if any difference in the parts lists for the 30 or 35. It has been said on the net that there may not be any difference at all. If you put on the bigger carb, you will need to go up two inches in pitch on the prop.

I have a 1988 Johnson 20 HP. First I removed the intake restriction and reused the 20 HP carb. The difference was noticable. Then I replaced the carb with one from a 35 HP. I also went from an 11" to a 13" prop. The difference was substantial. Before the mods the boat would not plane with the 13" and the motor bogged really bad. The 11" prop made the performance acceptable, but it was not spectacular. After the mods this thing will fly with the 13" prop. I think you could ski behind it.

One other thing to note is that sometime in the mid 80s they went to through hub exhaust on the 20s and 25s, while the 35s had it in the 70s. I think that the through hub exhaust will gain a couple of HP.

Some people will tell you, like they did me, that this won't work or there is more to it than this, but my experience says that the smaller motors can be upgraded with a carb change and a little work on the intake manifold.

Re: 1981 25hp vs. 35hp specs

yea, I heard of this mod being done on 9.9 to 15hp.
I was curious if the same was possible on other motors.
Thanks for the info.
jasper

Re: 1981 25hp vs. 35hp specs

Its a different mod on the 9.9-15hp. The carb swap was only good for the first few years (1974-76 or so).

The 20-25 horse went to thru-hub exhaust in 1985. The 35 hp was always thru-hub (1976 and on). The 35 hp became the 30 hp once OMC started rating hp at the prop in the late 80's. Therefore, you can only get 30 hp out of that motor with the mods mentioned above. Still worth doing, but check the parts catalogs at the BRP site for differences in the powerheads before you start.