That is a symptom of a LEAN mixture - not a rich one. Back out the LOW speed screw 90 degrees and try again. It should accelerate smoothly to full speed. If it does, listen to the engine note.
If it breaks into a 4-stroke at high revs, screw in the HIGH screw 45 degrees and try again. Keep going until it runs at full speed as a clean 2-stroke. There should be little or no visible smoke.
If starts to accelerate, then dies, or won't reach full speed, back out the HIGH screw 45 degrees and try again.
phil_saunders(a)bigfoot.com
Phil Saunders, June 2012