First check the condition of the air filter. It sounds like you're a little lean at low RPMs. Try opening the air screws on the carb about a 1/4 turn and see if it does better. If it gets worse go the other way a half turn. The carbs might be a little dirty or out of sync also. Kinda hard to tell because it may be an ignition miss at low RPM's also. Check your spark plugs first there. Real tough to tell if you haven't rode the bike.