I've got an old Yamaha 20 hp model 6A9 which is very little driven but now we have faced a problem which came during the winter in storage and I can't solve it. Problem is that the motor won't rev up high anymore.

This is what is done by far:

New spark plugs, new fuel pump, new CDI-box. I have cleaned the whole fuel line from tank to carb. Carb is ultra sonic washed and seems clean, also sprays. I also have tried to feed the fuel straight to carb with pump to be sure that there is enough fuel feed.

I have also checked TDC, advance ATDC is about 4 deg. and with high speed 24 deg. The butterfly open correctly and I also took off the flywheel and found nothing special. No broken wires etc.

I just don't know what else to do or check ? The engine starts very easily and idles well and revs up to maybe 2500 rpm. Both cyliders work well separately if you take the plug wire out and spark jumps easily over 12 mm.

Intake flaps are also perfect.

Can a coil cause this problem or leaking crankshaft seals ?

I'm pretty near to give up....


Jussi