Try a 5x3 cox prop and see if that helps. I will start out with a 6x3 Master Airscrew but if that doesn't work out I'll go to a 5x3 cox. I have used a cox 5x3 on my Black Widow and the engine really screamed but I don't think the plane was any faster. I have never used a cox 6x3 but it does look large for that engine. The Master Airscrew props are very narrow and not as fat as the cox.