There are 2 screws on the carb one in line with the throttle cable this adjusts the idle speed. The other one towards the back of the carb adjusts the air mixture. Before adjusting either of these make sure air filter is clean and spark plug is gapped properly.

This has a good write up of how to adjust the air screw: http://dirtbike.off-road.com/dirtbik....jsp?id=265950