Understanding the Third Idle Adjustment Screw on Rochester 2G Carburetors

Some Rochester 2G and 2GC carburetors are equipped with a third idle adjustment screw located on the rear of the carburetor. While most 2G models feature the familiar two idle mixture screws on the front throttle body (one for each barrel), this third screw plays a different role—it controls idle air bypass, not fuel mixture.

What Does the Third Screw Do?

This rear-mounted screw is commonly referred to as the idle air bypass adjustment screw. It’s located in the float bowl casting at the back of the carb, positioned vertically or slightly angled, depending on the version.

Its function is to regulate the amount of air bypassing the throttle plates at idle, helping to fine-tune idle speed and mixture in tandem with the front mixture screws.

How It Works:

Turning the screw in (clockwise) reduces bypass air, decreasing idle speed.
Turning the screw out (counter-clockwise) increases bypass air, raising idle speed.

It acts as a supplementary idle speed control, especially useful on engines with modified vacuum or where the curb idle screw alone doesn’t offer enough range.

Adjustment Procedure:

Warm up the engine to full operating temperature.
Set the curb idle speed and front idle mixture screws using standard procedure.
Locate the rear idle air bypass screw on the float bowl.
Turn the screw inward ¼ turn and wait a few seconds for the RPM to stabilize.
Continue turning in small increments until the RPM just begins to drop.
Then, back the screw out ¼ turn to return to optimal idle.

Repeat if necessary, always giving the engine a moment to react to changes.

Important Notes:

This screw does not replace the idle speed screw on the throttle linkage—it fine-tunes idle airflow.
It's especially useful when adjusting for cams with low vacuum, or on emissions-calibrated carbs where more precise idle control is needed.
If adjusting this screw has no effect, your idle bypass circuit may be clogged.