And…what is the difference between the 25/35?

[johnyrude200]

Rehabbing an ’82 model 35hp, and I pull the carb to find the manifold with a quarter-sized orifice. So I go to M.E. and also OMC system match parts lookup, review both sites and both 25 & 35 diagrams, and the only difference I’m seeing in the manifold are the leaf valve stops.

Now granted the 25 does not have the thru-hub exhaust, but the carb is the same for both the 25 and 35 (orifice high speed jet is 59D). Is that all it takes to up the HP by approx. 10hp…more breathing of the motor from the thru-hub exhaust?

If you look at newer versions, even just a couple of years, you see the manifold orifice is much larger, in fact the old 18-20-25hp motors from the 70’s have manifolds with much larger orifices. Granted that is a totally different block, but interesting nonetheless.

[garry-in-michigan]

That tuned exhaust helps a LOT. And does it without to much more fuel … 😀

[dan-in-tn]

Block is different. Double drilled exhaust ports for one and I think maybe Siamese intake ports. The size of the HS orifice is not the only thing that decides how much fuel the engine get at WOT. The nozzle determines fuel flow also. Like Gary says you can’t under estimate the importance of the thru hub exhaust either. Tremendous advantage in horsepower development.

Dan in TN

[bluehaze]

I have a few of these motors from the 1990’s. 20hp, a couple 25hp’s, and 30hp.

Within the same year, the only difference I’ve found, is the intake manifold and the carbs. Everything else has the same part numbers.
Differences between model years can be found though.

[fleetwin]

I’m seeing different part numbers for the 82 25 and 25 models, did just look up the 15" rope start models though…..

[bluehaze]

Hmm. Didn’t see he was referring to the 1982 model. I was looking at the years I have in the mid 90’s. Within those years, the block, exhaust, cyl heads, pistons, cranks, etc appear to be the same.

[frankr]

Regardless of horsepower, the thru-hub gearcase is faster than the old one. Bigger but faster. Gotta be the shape.

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