Airraptor, thanks for your thoughts on the prop selection. I was told that it is a long stroke engine and works well with bigger props at lower rpm.

I managed to dig up an old RCM review of the engine by Clarence Lee, in which he quotes a 9500 rpm on a Zinger 14x7 prop, using the tuned muffler and 15% fuel. Assuming an rpm increase of about 400-500 by using the tuned muffler over the conventional one, my engine is still a 1000rpm short. I will try and run it this weekend, weather permitting, on 15% fuel and lean it out a bit. I am a little hesitant though about the 15% nitro as I hear they don't like anything above 5%.