YS63 problems!
It's hard to diagnose problems like this by wire. It is possible that the crankcase might have a pinhole in it which will cause this type of problem.
The only way to get this corrected properly is to send the engine to our service dept.
YS Service
1295-C Industrial Court
Gardnerville,
NV 89410
Be sure to include a note outlining to problem.
Thanks,
Dave