The idle screws on a Rochester Quadrajet carburetor fine tune the air-fuel ratio going into the engine during idle. The most common cause of rough idle quality in engines equipped with this carburetor is an unevenly adjusted idle mixture circuit caused by incorrectly adjusted mixture screws.
Start the engine and allow it to run until the thermostat opens and upper radiator hose becomes hot. Many Quadrajets are computer-controlled, and operating temperature must be reached before the idle mixture can be accurately adjusted.
Turn the engine off. Bench adjust the mixture screws by using the double “D” tool to screw the mixture screws, located on the front lower edge of the carburetor, into the carburetor until they lightly seat. Unscrew the screws two and one half turns.