Are ya sure its not flooding itself? Off the top of my head it "sounds" like the LSN (low speed needle)is a little toorich.. What does the "pinch" test tell ya about the LSN? (Pinch test: Once it's up to operating temp and idling, pinch the fuel line that enters the carb until the motor dies.. (If the lsn is set correct, it should rise in rpms slightly, and die within a second or 2.. If it takes longer than 2 or 3 seconds to die, the LNS is too rich.)If it is too rich,LEAN (clockwise) the LSN 1/12 of a turn, do a lap and let the engine return to idle, then try the pinch test again.. Do this until the rpms rise a bit and the motor dies with 2 seconds.. Once the LSN is set, if it still does it we'll try something else.. Hopefully it's something simple like that though..