The most common mistake I see with beginers and OS engines, is trying to run to big a prop and make engine have high pitch whine. All your doing is leaning out engine to a point it's not going to fly. Most all of my engines are OS and I have learned the literature they give you about prop size that you can run is not true. I have OS 75 on 10# Spade Plane my son built, 12x8 prop 11,000 RPM. This combination flies well.