Well after some experimentation with my Ryobi 31cc conversion, I finally got it running well. I posted a while back about it unloading and 4 stroking in the air.
I realize now that with this gasser anyway, you have to let it choose the prop and not the other way around. Normally with my glow motors I prop the engine to get a certain desired performance. I realize now that with this gasser you have to let it choose the prop to load the engine correctly. Of course this is a catch 22. You have to load the engine to keep it running cleanly, but not load it so much that it has no power. I settled on a MAS 18x10 turning 6400 rpms on the ground. Performance was really pretty good in the air ! I got the additional top speed I wanted out of the plane. Alas, with top speed comes flutter. Stripped out one of my aileron servos. I'll have to resize the ailerons...they are really too large. It was challenging to say the least trying to land with a floppy aileron in a cross wind.
Enough rambling...just wanted to share what I discovered about my conversion motor. If anyone out there has the same predicament, try loading the engine a little more with a larger prop.