That is kind of what I was thinking, sounds like your pilot is running a little lean. If the machine sat for a long period, then carb buildup could definately be the problem. If it just started happening all of a sudden, and the machine has not been stored for a long time, I would check for an air leak in the intake track. If you just aquired the machine, or made a significant change in elevetion, I would try adjusting the pilot screw or going to a larger pilot jet.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.