I think I've got it Dave, after I re-built the tank to get rid of the leak I have a steady top end on my FZ91. The idle is about 2700rpm and the top is 10,000+ with a 15x6 break-in prop, I richened it up to 9700. Using Cool Power 20% heli fuel. It is in a Balsa USA Nieuport 17, hope to fly it this weekend.

Lot of vibration at the low end, what is good to put on the engine mounting screws to keep them from backing out?

Use blue loctite if you must, and put a 15x8 on it as well.

Good Luck!