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▷▷ 2021 ▷ How to adjust the idle screws in a Quadrajet carburettor

4 julio, 2021

How to adjust the idle bolts on a Quadrajet carburettor

by Lee Sallings

Image of american v8 engine from kenmo from

The idle bolts on a Rochester Quadrajet carburetor adjust the air-fuel ratio that enters the engine during idle. The most common cause of poor idle quality in engines equipped with this carburetor is a poorly adjusted idle mixture circuit caused by improperly adjusted mixture screws.

Step 1

Start the engine and let it run until the thermostat opens and the upper radiator hose warms up. Many Quadrajets are computer controlled and the operating temperature must be reached before the idle mix can be fine tuned.

Step 2

Turn off the motor. Bench-tighten the mixture screws using the double “D” tool to screw the mixture screws, located on the lower front edge of the carburetor, into the carburetor until they are lightly seated. Unscrew the screws two and a half turns.

Step 3

Starting 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 necessary for the engine to start roughly again. Screw the idle screw back into the carburetor half that number of turns. Repeat the process for the inactive screw on the right side.

Test the vehicle to verify that the engine does not stall or load at stops. Adjustments to fine tune the idle quality should be made in quarter turn increments and should be made on both idle screws equally to maintain idle circuit balance.


  • If there is no difference in engine idle speed when the bolts are turned, you may need to rebuild the carburetor.

Articles will need

  • Double “D” carb setting tool

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