Holley 1920 Idle System

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.

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.

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.

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.

On the Holley 1920 (a classic 1-barrel carburetor commonly found on Chrysler Slant-6 engines), the transfer slot acts as a bridge between the idle circuit and the main metering system.

Think of it as the "hand-off" point that keeps the engine from stumbling when you transition from standing still to moving.

Key Functions

• The Transition Bridge: When you are at a stop, the throttle plate is nearly closed, and fuel comes only from the tiny Idle Discharge Port. As you press the gas pedal, the throttle plate swings open and uncovers the Transfer Slot.

• Preventing Hesitation: Without this slot, there would be a "lean spot" (a moment where the engine gets too much air and not enough fuel) before the engine speed is high enough to pull fuel from the main nozzle. The slot provides that extra fuel needed during the first few degrees of throttle opening.

• Supplementary Fueling: It uses the same fuel "well" as the idle circuit, but because it is a long vertical slot, it progressively reveals more surface area as the throttle opens, smoothly increasing the fuel flow.

Tuning Tip: The "Square" Rule

If you are looking at the bottom of the carburetor with the throttle closed, the amount of the transfer slot visible below the throttle plate should look like a small square (roughly .020").

• Too much slot visible: Your idle speed is likely set too high by the screw, which "robs" the idle mixture screw of its effectiveness and can cause a rich stumble.

• Too little slot visible: The engine will likely hesitate or "flat spot" the moment you touch the gas because the fuel won't start flowing quickly enough.