Many gas engines recommend breaking them in on the airplane rather than a bench because of the additional vibration. Gas engines by their nature will produce more vibration because the gasoline burns faster in the cylinder. It is therefore more critical to properly balance the prop as well as make sure your electronics are properly vibration mounted. Recievers and batteries wrapped in foam, servos not mounted so tight the grommets are crushed, etc. Engine mounts for gas engines are also designed to use the airframe to absorb vibrations. Bench mounting may overstress the engine mounts and lead to premature failure.

Brad