Originally Posted by Dave Campbell
I am turning an APC 6.3-4 at about 16,500 on 30%, which is about 1k short of what the guys on RCU say I should get when the engine is worn in.
You might have to much nitro in the fuel, mine does +17000rpm on that prop and 10% nitro. Check the number of shims and optimize for top top revs, staying on the side of slightly too low compression.
Another thing to look for is the throttle barrel, it is important that it lines up with the carburator bore at full throttle. It can be off sideways, as it rotates on it's threads, or it might not open fully if you have the throttle stop screw still installed (just remove it, it is not needed).
A cox babe bee will do more that 12000rpm on a 6x3, there is certainly something seriously wrong...
Take off the engine from the plane and run it in on bench before you put it in a plane.