12 x 5??? That seems a bit large to me. We use a 10.5 x 6 as the smallest prop. A 10 x 7 or 10 x 8 are goodies for trainers or other sport aircraft that aren't intended for 3D-type flying. If you prop the engine so that it turns 12,500 RPM or above when leaned to just rich of peak, you'll find the engine performs very, very well. However, it will be quite a bit noisier than if you run larger propellers.