Holley 1 Barrel Choke System

Holley 1920, 1 Barrel Choke System

A Holley 1920 divorced choke refers to a specific type of automatic choke mechanism used in some Holley 1920 carburetors. In this context:

Holley 1920: The Holley 1920 is a model of carburetor produced by the Holley Performance Products company. Carburetors like the Holley 1920 are used in internal combustion engines to mix air and fuel for combustion.
Divorced Choke: The term "divorced choke" refers to the arrangement of the choke mechanism. In a divorced choke setup, the choke housing and the carburetor body are separated or "divorced" from each other. This means that the choke housing is mounted separately from the main body of the carburetor. It typically has a bi-metal spring that expands as the engine heats up, gradually opening the choke plate to allow more air into the carburetor and lean out the air-fuel mixture.

The purpose of the choke mechanism in a carburetor is to enrich the air-fuel mixture during a cold start, providing a richer fuel mixture that helps the engine start and run smoothly until it warms up. Once the engine reaches a certain temperature, the choke gradually opens to allow more air in, adjusting the mixture for normal running conditions.

The term "divorced choke" is used to distinguish this setup from other choke types like "integral choke," where the choke housing is directly integrated into the carburetor body. The choice of choke type can depend on the specific carburetor design and the requirements of the engine it's used on.

Convert your divorced choke to an electrical choke. Parts for the divorced choke is becoming increasingly harder to find.

Holley 1920 Choke Parts

The Holley 1920 is a classic one-barrel carburetor commonly found on Chrysler "Slant-6" engines. Adjusting the choke involves two primary settings: the Choke Thermostat (Spring) and the Choke Vacuum Kick.

Before starting, ensure the engine is cold for the thermostat adjustment and that you have a vacuum pump or a way to safely apply vacuum for the pull-off adjustment.

1. Choke Thermostat (Automatic Choke)

This adjustment controls how much tension the bimetal spring applies to the choke plate when the engine is cold.

Locate the Choke Coil: On most 1920 setups, the choke thermostat is a "well-type" coil mounted directly on the exhaust manifold, connected to the carburetor by a rod.
The Adjustment:
- Loosen the mounting nut on the choke coil housing.
- To lean the mixture (choke opens sooner), rotate the housing so the rod moves toward the "open" direction.
- To richen the mixture (choke stays closed longer), rotate it toward the "closed" direction.
The Standard Setting: Usually, there are index marks on the housing. Set it to the center mark or one notch "Rich," then tighten the nut. The choke plate should just barely touch the closed position when the engine is stone cold (approx. 21°C).

2. Choke Vacuum Kick

This is the most critical adjustment for preventing stalls immediately after a cold start. It "kicks" the choke plate open slightly once the engine fires and creates vacuum.

Apply Vacuum: Use a handheld vacuum pump to apply at least 10 inches of Hg to the choke vacuum diaphragm.
Measure the Gap: Push the choke rod toward the diaphragm to compress the internal spring. Measure the distance between the top edge of the choke plate and the air horn wall using a drill bit of the specified size (usually between 2mm and 4mm, depending on the specific year/application).
Adjust: Bend the small tang on the choke linkage or turn the adjustment screw on the diaphragm linkage until the gap matches the specification.

3. Fast Idle Cam

While adjusting the choke, you must also ensure the fast idle speed is set so the engine doesn't stumble while warming up.

With the choke fully closed, the fast idle screw should be resting on the highest step of the fast idle cam.
Adjust the screw to achieve the RPM listed on your vehicle's emissions decal (typically 1,500 – 1,800 RPM).

Quick Tips for Success

Check for Binding: Move the choke linkage by hand. If it feels "sticky," clean it with carburetor cleaner. Do not oil the linkage; oil attracts dust and will cause it to gum up again.
Vacuum Leaks: If the vacuum kick diaphragm is ruptured (won't hold a vacuum), the engine will likely "load up" with fuel and die shortly after starting. Replace the diaphragm if it leaks.