My brief testing in the air seems to leave me thinking the 20x8 is the prop. I tried it with a 20x6 on a quarter scale Kat and it just seemed like I had to really keep the revs up. Also on the ground at full throttle, the RPM was a bit high. I didn't record what it hit. Now, if I could just get my Futaba equipment to be as reliable as my CRRC 40cc, I might actually be able to keep something in the air long enough to really test the engine. It has run like a top throughout breakin on the ground and in my brief times in the air. It's scheduled to go on the third model now.
Meanwhile, my radio and all planes are grounded until that gets sorted out.