If this is the GRRT with the rear tuned exhaust/rear intake with the 11mm bore venturi (wide open throttle all the time-stock) or 9mm throttled carb, it should be turning in the 20K+ range easy. This is an FAI Pylon Racing engine! RPM=POWER. Mine is set at 22,000 on the ground (really smoking rich) so I don't burn plugs (as much) as it unwinds in the air. Turning a 7x9 prop. You have it WAY over propped!! It's tuned to run up there...max power is at 25~26K. Running it there though will burn it up quicker (plugs too...WOW!) than at 22K or so. Also, WEAR EAR PROTECTION!! This engine is LOUD when revved..but man, does it sound AWESOME.
Got mine on a Dust...wow.
Also, if ChuckAuger reads this...I FINALLY GOT MY AIRSPEED INDICATOR!!! Email me and get back in touch! I'm gonna build a new Dust this winter to put it in though...I've worn out my current one pretty well.
Carl