Adjusting the Idle Mixture

The Carburetor Idle Circuit and Adjusting the Idle Mixture

The idle circuit is in effect only when idling. Once RPM increases above idle, the Idle mixture screws are no longer in play. If you have the idle RPM set too high, your idle mixture adjustment will be irrelevant. When you turn the idle mixture screw in all the way and the engine doesn’t change, you could have the idle RPM set too high. For example if it was at 1,000 RPM. On the other hand if the idle is normal, then you have a problem in the carburetor, or possibly a vacuum leak.

The idle mixture screws sets the mixture of fuel and air during idle RPM.

Single barrel carburetors will have one idle mixture screw while 2 barrel and 4 barrel usually have 2 idle mixture screws.
You can clean your idle mixture screws by buffing them off with a wire wheel.
Inspect the screw for grooves. Grooves are created when the screw is turned in too tight. Replace any damaged or bent screw.
Inspect the screw hole to make sure it is clear. When you blow through the hole you should get air inside the bore.

For Rochester Quadrajets pick up a idle mixture adjusting tool at most any part store. The tool bends which will make adjusting easier.

When assembling the carburetor turn the idle mixture screws in all the way, gently seat, then turn it out about 1 1/2 turns.

Bring the engine up to operating temperature.
Make sure the choke valve is completely open.
You may have to rev the engine slightly so that the fast idle cam moves to the idle position.
Adjust the idle to specification.

There are a couple of ways to adjust the idle mixture.

1. Using a vacuum meter
Hook the vacuum meter to one of the vacuum ports on the intake, or the carburetor.
Take turns with each idle mixture screw.
Turn each screw out a bit for a start (maybe 1 turn).
Turn each screw in 1/4 of a turn and wait for a second for the vacuum meter to catch up.
Do this until you get the smoothest idle and the vacuum meter stays steady.

2. By ear
Take turns with each idle mixture screw.
Turn the screw out 1 turn to start.
Turn the screw in 1/4 and wait for a second for the engine to catch up.
Keep doing this until the RPM starts to drop.
Turn the screw back 1/4 – 1/2 turn.

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