Some Rochester 2G carbs have a third idle adjustment screw on the rear of the carburetor (normal idle mixture screws are located parallel to each other in the throttle body on the front of the carburetor). This screw regulates the amount of air that is supplied to the engine by the idle air by-pass channel. The idle air adjustment screw is mounted in the float bowl casting at the rear of the carburetor. Turning the screw inward (clockwise) decreases the engine idle speed and turning it outward (counter-clockwise) increases the engine speed.

Adjust it just like you would the idle mixture. Turn in 1/4 turn, let rpm catch up, When RPM starts to drop, turn back out 1/4 turn.