1. Check compression - should be 65psi minimum and even across both cylinders.
2. Change water pump impeller.
3. Check coils, points, condensers, plugs, wires.
4. Clean/rebuild carburetor.
5. Change gearcase oil - check for water intrusion.
Go to the Top Secret thread at the top of the forum - it will provide you with lots of good information.
Guy did whole series on it on Youtube I found.
65psi minimum and even across both cylinders.
It was crap will soaking cylinders in oil for week help, too bad it was real clean looking, but got it free.
Low compression if present can often be fixed by resurfacing the head & replacing the head gasket. If the cylinders aren't too worn you could also give a fast scuff to the cylinder walls and install new rings; that would also help with compression. Perhaps the rings are just sticky and allowing blowby; a good old cleaning out by running a strong batch of Seafoam in the fuel might free up the sticky rings and restore compression. Try that before ripping into the motor.
