Sorry I don't have my notes on RPM figures with various props. The 650 is well worth your time. Definately convert it.

I flew a bunch of engines on a Bridid Big Bee. The CS650 was one of them. I thought I might get 30% more power than my Echo CS 1080 50cc. The 650 spins a 22x8 Bambula at over 8K. That sounds real good, like 40# of thrust. Thing is that the BME 50 clone is no slouch either. The 50 is quite a bit lighter. The 65 weighs 6#. The 8K figure is with a big HDA carb and an open exhaust. When you put on a smaller carb to get a good slow idle the top end suffers. I fly off asphalt so I need the slow idle. I found that the 50 gave similar performance due to the lighter weight. The airplane also seems to be a bit more aerobatic with the 50. I tried bigger props, on the 650, and did not get better pulling power as measured by a scale tied to the airplane tail. I would also need to make taller gear to avoid trashing props. In the end the 650 was a lot of fun. It gave crazy power and a lot of satisfaction. I also spend a lot of time tweaking it. I never was fully satisfied with how it ran.

The jug is turned 180* in the photos. If you do that also turn the piston. I have been making a ring for the magnet on the hub that can be ajusted lately. I like that a lot better than how I did this one. I have been pluging the back of the case with a turned cup and cutting the crank. I also like that refinement.

You know, the scooter engine guys have a Zenoah G-62 short block. That one is high on my list of projects. I'd do a a G-62 Lite type of thing. The G-62 has a great reputation.