I have a five year old .91 that has only recently been run about 2 hours, been broken in properly and flown a dozen times in a Hogbipe with great success. It now starts easily, idles perfectly and runs well up until just about full throttle when it backfires and stops. Big change in operation with no root cause visible. On the rich side for high speed needle, no visible tank or line leaks and the valves appear to be properly adjusted. Some small resistance in crankshaft apparent when slowly turning by hand but assume that is cam operating. Very frustrating and hoping someone can help before I send the thing back for factory help.