The idle fuel is fed through the main jet through the idle well, then mixed with air through the idle air bleed.

The idle air bleed is a very small passage. Be sure the air bleed is clean and clear.

Holley 1920 Idle

The fuel air mixture travels down the idle passage and out the idle discharge port. The idle discharge port is located just below the throttle valve.
As the throttle opens and before the accelerator circuit takes over, there is a need for additional air fuel mixture. As the throttle opens it exposes a transfer slot located just above the idle discharge port allowing more air fuel mixture.
If you are experiencing a bit of a hesitation just off idle, then check the transfer slot to be sure it is open.

Adjusting Idle

While on the bench screw the idle screw clockwise until it bottoms. Do this gently. idle screws are damaged by over tightening.
When on the car, bring the engine up to operating temperature.
Adjust the idle.
Turn the idle screw clockwise 1/4 turn at a time hesitating between turns to allow the engine rpm to catch up.
When the rpm starts to drop off, turn the screw counter clockwise 1/4 turn.

Adjusting the Idle Mixture Doesn't change RPM

This is indicative of a carburetor problem, assuming you are adjusting the mixture at idle. Once over Idle, the idle circuit is not used. The problem could be a vacuum leak, or clogged passages. Take a close look at any of the small passages that lead to the top of the carburetor. One will be the idle air vent as shown above.




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