ORIGINAL: Bax
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.
I was running an 11x5 for the first half of the summer, getting no more than 12,500 rpm. At one point I tried the 12x5 and it was a little too much for the engine. I kept the 11x5 for a little while longer, then tried the 12x5 again and got over 12,300 peak RPM. The engine sounded and ran great, and the prop did very well on my Venus .40 and my Phoenix Dolphin.