Did you flip the carb from the standard oem position?, if so look for the little hole that runs from the crankcase into the carb, when i flipped my carb this line was cut off and it would run great on the ground then deadstick in the air. If you looking to save some weight you could convert that engine to gas/glow, only an adapter and a 4 stroke plug is needed, it's well worth the weight savings.
Here's my little converted weedeater http://www.rcgroups.com/forums/showthread.php?t=793079