Ditto on the prop...I had two these engines and they did best in the 18 in. range. I suspect you have an air leak in the crank case. These are mostly "kit" engines to be assembled by the purchaser,
As such many folks have never assembled and sealed any engine before. Both of the ones I had leaked air and had to be re-sealed on the bottom end. I was not really impressed with these engines
for reliability, quality and power and got rid of them.
Others report good results.............
