sounds like its running to rich, check your air filter is clean, set your carb mixture ,
turn the screws in untill they bottom out ,but dont turn to hard !(they have a fine needle point end and are easy to break leaving the broken point inside the carb making adjustment impossible )back all four screws out 1.5 turns and start from there.run the bike and adjust from there to rich turn out to lean ( weak turn in )
www.thegsresources.com a good web site for these bikes.
good luck