I'm using an OS F plug and the engine is mounted inverted. I can't put a larger diameter prop on due to ground clearance, but I do have a 2-blade 11x9 and a 3-blade 11x7 that I could try. I live in Europe where nitro is horrendously expensive, but am returning to the states in August. I have some Klotz synthetic oil I could add. The instructions say the valve gap should be 0.1mm which is a hair under .004 inches. I'll see what I can find to get the distance correct.

This has definitely been a learning experience.