The carb usually has a set screw on the side of the throttle body that governs the throttle opening for the engine to run idle. Look at the carb from the left side of the bike, there are two screws. The one on the right (closest to the air intake) should be turned all the way in - gently! - and then one and three quarters of a turn out. The one on the left (closest to the cylinder intake) is the idle adjustment. Officially it should be set so that the engine will idle at 1500 rpm. If you don't have a rev counter, set it so the engine idles smoothly with a reasonable safety margin before it engages the clutch.