Fuzz,

When we run the O.S. Max .61 FX with an APC 12x6 prop, we expect to see it turn in the mid-12,000's. If your engine is running slower, it's either damaged from something, or was bad out of the gate.

The usual cause of damage is an accidental overheat, caused by an unitentional lean run. You'd need a new piston/cylinder assembly to repair the engine.