1969-77 18-35 hp Evinrude/Johnson parts interchange

[Old runabout]

I’ve got a ‘69 20hp Johnson (20R69B) manual start on my 14’ Lakefield. Runs well, fresh carb rebuild, new ignition, water pump, clutch dog....but I really would like electric start and wouldn’t mind a bit more power.

I just acquired a complete but non- running ‘73 25hp evinrude (25352AC) electric start and a ‘75 25hp evinrude parts motor (25552BC), with some obvious parts swapping having taken place, so it’s already a bit of a mongrel.

Ideally I’d like to put together a solid 25 hp with the electric start, but wondering if it’s possible to swap the key parts onto my running ‘69 (plus, I prefer the look of the Johnson engine cover)

My questions, for someone that knows this family of motors, are:
-what is determining the power output? Carb? Displacement? Could I swap the carb from one of the 25hp onto my 20 (and get 25hp?). If it’s not the carb and jetting that determines the hp difference, what parts do?
-gear case and prop shaft: the ‘73 parts motor has a splined shaft, which as far as I can determine is not a stock setup for a 25hp motor; is this splined shaft possibly from a 35 hp motor of the same vintage?(possibly from ‘77?)...other than the strength advantage of splined vs pin drive, is there a compelling reason for me to swap in this splined shaft (or the complete lower unit)? The diameter and pitch of possible props is the same.
-engine codes: what does the ‘C’ suffix indicate? Commercial? Canadian market?

Thanks for any insight you can offer

Pete

 

[racerone]

The later models used a splined propshaft.---Those splined lower units will fit onto older motors.----All electric start bits from 22 cubic models will fit from about 1958 to 1976.----The C means very little other than some secret mid year change to the model.

 

[F_R]

Many parts can be swapped on the 22 cubic inch motors. But don't be fooled. They made them so it isn't that easy. The 25 is a souped-up 18-20, true. But the 25 has different pistons, crankshaft, cylinder head, and other stuff. Unless you change ALL the affected parts, you will just muck it up. and likely destroy it. You certainly would not be the first one either.

 

[jimmbo]

One thing. DO NOT put the 25 cylinder head on the 20, unless you want to destroy the 20. When OMC hopped up the 18/20hp into the 25, they effectively changed the port timing by using pistons that had the wrist pin closer to the crown of the piston. This effectively placed the piston deeper in the cylinder at TDC, which lowered the compression ratio. A higher compression head restored the compression ratio. When used in conjunction with the 20hp pistons this head raises the compression too high resulting in detonation

 

[Old runabout]

Thanks for all your replies and wisdom, that’s exactly the level of detail I was hoping for. So, it seems I can either convert the 20 to electric start and carry on, or revive the 25 and live with the boxy looking evinrude engine cover .