Mike - I have only used a 15 X 6 APC prop on this engine in this airplane. Previously, the engine was mounted in s Super Sportster 60. In that plane, due to height restrictions, I ran a 13 x10 APC prop and didn't experience any exhaust problems.

I did contact Performance Specialties (YS Engines) and they thought that either the engine exhust was getting too hot ( not enough air flow) or that at high RPM ( i.e. > 9,000) some sort of harmonic vibration was occuring. I believe the engine is getting plenty of air flow, so I'm going to experiment with different size APC props starting with a 14 X 10.