There was another thrend in RCU covering this very topic. I also was having this problem. An easy fix suggested was to increase the fuel flow by using large I.D. fuel tubing for fuel draw to the carburator. This advice was taken and the problem went away. My engine is swinging an 18x8W APC propellor at 8100 rpms'. Fuel is 10% and the glow plug is a #8 O.S..