Thanks Alan. I've had an 18x8 on the engine with the same results. I do find it odd that the Saito link that you provided recommends different prop sizes than the engine manual. The manual actually lists the 15x8 as the starting point for the 1.80 which I always found odd since I run a 15x10 on my 1.20. I'll check the valve clearance and try it again.

Thanks again!