Sounds like it may be running very rich which lets the engine start ok when cold but getts boggy when warm. Clean the airfilter in soapy water, dry thoroughly, turn the "H" setting all the way in and then anticlockwise by 1 turn, fire the beast up and warm it up, on full throttle get the setting so it really revs up freely and then turn anti clock wise till the engine just burbles slightly, this will only be half a turn to one turn.
Hopefully this will work, if it doesnt then it may be a partially blocked jet:-(
Steve, March 2009