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How to Adjust Idle Screws on a Quadrajet Carb

by Lee Sallings

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.

Step 1

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.

Step 2

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.

Step 3

Start the engine. Screw the left idle screw into the carburetor until the engine starts to run rough. Slowly unscrew the screw and count the number of turns it takes to make the engine run rough again. Screw the idle screw back into the carburetor one half that number of turns. Repeat the process for the right side idle screw.

Test drive the vehicle to verify that the engine doesn't stall or load up at stops. Adjustments to fine tune the idle quality should be done in quarter-turn increments, and should be done to both idle screws equally to maintain the balance of the idle circuit.

Tip

  • If no difference in engine idle speed is caused when the screws are turned, you might need to rebuild the carburetor.

Items you will need

  • Double "D" carburetor adjusting tool

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