Thanks. I just ran it again with a #8, a bit more castor and an 8x4.

This is just a sick engine. The higher the rpms go the more it shakes and vibrates. At full throttle the rpms waver up and down and it sounds like it's getting ready to blow apart. The engine will shake violently for a second or two, then run smoother, then shake again, and so on. There is definitely a mechanical problem inside the engine. I think the larger prop was acting like a flywheel and damping some of the vibration. With the 8x4 it goes psycho.