If adjusting one or both of the idle mixture screws does not make a noticable difference, something is wrong.

Turning the idle mixture screw all the way in, should lean the carb out and cause the engine to die. Turning the idle mixture screw out will with richen the mixture.

What to check:

• Look for blocked passages in the idle circuit.
• Check the venturi passages carefully.
• Run fine wire or nylon brushes through all the passageways.
• Check the idle mixture screws for bent ends or groves in the taper. If either appear, you’ll need to replace them.
• Look for vacuum leaks by spraying carb cleaner around the base of the carb, around the throttle shaft, and at any vacuum line connection. If you hear a change in RPM, you’ve found a vacuum leak.