They are really a good engine. Since they are plain bearing and cheap, people expect them to break-in in 2 or 3 flights like the OS LA's. They take considerable time to break-in-it's the sleeve bearing, but perform well afterwards. A 12-6 is a good prop for them. They tend to not idle well, or slowly, when new, but I think this is caused by drag from a tight sleeve bearing.

The GP .61 has a simple air-bleed carb with a fairly small throat. If it had a fuel metering carb and was broken in well, it would out perform the Pro, in my opinion. The GP is really a .65, as I recall and the Pro is a .61. Cubic inches.