Sounds like time for a good carb cleaning then. Could be crap trapped in the high speed circuit which would kill the engine after about half throttle. A rebuild kit costs about $10.00 and takes about half an hour or so to do. Pick up a can of spary carb cleaner and a can of compressed air for cleaning computers and such and clean everything out. Replace gaskets and diaphragms while in the process.

They are quite easy to do. Just track the order the parts come off and replace in reverse order.