OMC 25hp 32 C.I. Flywheel compatibility

[johnyrude200]

I have a 1981 25hp motor that I just observed something interesting on I thought I’d share, and also ask further explanation for.

I put a 25hp flywheel on this motor because there were 4 different flywheels used on the 25/35hp motors between production of 1976-2006.

1. Low tension magneto style, no manual starter option (1976-1978ish?)
2. small diameter gear flywheel with big diameter starter bendix
3. med diameter gear flywheel with medium diameter starter bendix
4. large diameter gear flywheel ‘pancake style’ with no manual starter option (starting in the late 80’s)

I have had plenty of success putting reconfiguring these motors to whatever a customer wants and switching flywheels between productions years (IE, go from remote control electric start to manual start tiller steer, requires flywheel change depending on year).

But ran into an issue with this last one. The 1981 25hp motor did not like the 2006 25hp flywheel. The motor would start and run fine, idle low, and have a normal power curve up to about 4200RPM. It wouldn’t go above this RPM, and would have high speed misses, and I can tell it was running very rich.

I ruled out the rest of the engine systems as root issues; changed out the ignition, switched carburetors, checke max spark advance with a timing light, etc. As a last approach I switched the flywheel back to a period correct one and the problem went away.

Why is this?

The ’04 flywheel is a pancake, flat flywheel with no option for manual start.

The ’81 era flywheel is heavier and has an option for manual start. Same tapers, magnets appear in the correct position.

[fleetwin]

Well, I know that flywheels did change over the years…Excluding the models using the points, I kinda feel as you do, that the ignition function would be the same. The changes were like you say, to accommodate different starters/recoils, etc.
Did you try putting the newer flywheel back on to see if the problem reappeared? If so, I would compare WOT timing between the two flywheels, then I would do cranking output tests on the sensor and charge coil using the peak reading voltmeter, then compare these readings to the newer flywheel.

[johnyrude200]

Ill check the markings on the flywheel to see what the degrees TDC alignments are. I will say I used a newer starter bracket but the timing light had it right on at 34 degrees.

I was just happy to hit 5500 RPM on the test wheel once I switched flywheels. I had 5 hours invested prior to that going through the other components to rule out other sources! Had to sleep it off because frustration was setting in. Fresh day, new perspective!

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