Powerhead swap

[Roonhunter]

I have a 1972 25hp Johnson 25R72R that needs rebuilt, which I planned on boring .030 over, I also have a 1965 18hp FD-19D. Can I take the power head from the 65 and put it on the 72, while rebuilding the 71 powerhead? Would I also need to use the flywheel from the 65? I had planned on using the carb off of the 72 as well as the armature and flywheel but the more I think of it it’s prob not possible or a good idea to swap flywheels....thoughts? Thanks in advance.

 

[F_R]

flywheels should be interchangeable as far as I know.

 

[Roonhunter]

Awesome thanks!

 

[Roonhunter]

Well damn, can’t use the carb off the 72, unless I change out the intake. Hole pattern is a little narrower on the 65....

 

[oldboat1]

Why not just rebuild the '65 carb, assuming you have it?

 

[Roonhunter]

Now that you mention it, it has been rebuilt. But it’s and 18hp, where as the 72 is a 25. I’d lose my man card if I left 7 more hp in the garage right?

 

[racerone]

On the 1972 model 25 HP there is more involved than the carburetor to get the extra power !!

 

[Roonhunter]

Interesting racerone, I was under the impression that the jetting was responsible for diff hp from the same 22ci powerheads. Now I’m really curious lol

 

[F_R]

Ha! That jetting rumor is locker room baloney. To begin, the 25 uses different pistons and cylinder head and you can't change one without changing both. Second, the 25 crankshaft is balanced because it turns at a much higher RPM. The entire crankcase drainage system is different on the '72 to lessen overboard emissions, which might complicate your intake manifold plans. Finally, there was a major exhaust housing change at some year, which might complicate your planned powerhead swap. Short story: If you want a 25, buy a 25.

 

[racerone]

The tall tales about increasing horsepower with a jet change just keep on coming.------I knew F_R would spell out the details here.------It takes more than 5 minutes to explain the knowledge that has been gained over many years and many motors!!

 

[Roonhunter]

Great info! I think I read the jetting info on leroys ramblings or whatever. I have the powerhead mounted on the exhaust housing, went together like butter. Also went ahead with the 65 carb as well although it appears as tho I’m missing a needle valve bushing. Everything thus far seems to be falling in place, hopefully I can check for spark and set timing after work. I appreciate the info yall!

 

[Roonhunter]

Update....She runs! Not a fan of the carb on the 65 tho, on the 72 I could adjust the butterfly on the arm, whereas on the 65 there is only the “tweaking” of the throttle cam. And that’s a bit of a PIA with it together..

 

[racerone]

I would say that the 1965 aluminum carburetor is far easier / cheaper to fix then some of the new plastic carburetors !