How Does a Carburetor Choke Work?

by Don Bowman


Carburetors have either manual or automatic chokes. The manual chokes require a choke cable to be mounted inside the car. There are two types of automatic chokes. The older style chokes have a coil spring mounted in the intake manifold with a rod from the spring to the choke. When the engine is cold, the spring is contracted and pulls the choke closed. When the engine heats up the spring expands and pushes the choke rod up, opening the choke.

How Chokes Work

The automatic chokes, which are the most common, work on a spring mechanism housed in a plastic cover located on the passenger side of the carburetor. These chokes are electrically heated to promote expansion and contraction. When the engine is cold the spring contracts, coiling more tightly. This contraction pulls on the choke rod and puts pressure on the choke to close completely.

When the engine is started, the closed choke creates more vacuum in the engine, pulling in more fuel. As soon as the engine starts, a vacuum is applied to the vacuum choke pull-down servo, which in turn pulls the choke open slightly--just enough to allow enough air into the engine to keep it running. As the engine continues to warm up, the electrical choke responds to the heater inside and begins to expand and as it does, it begins to push the choke open.


These electrical chokes have the ability to be adjusted. There are three screws around the cover of the choke spring housing. With the engine cold, loosen the screws in the cover. If the choke stays on too long or does not come all the way off, then turn the choke housing clockwise until the choke just begins to open and stop and tighten the screws. If the choke does not close enough, turn the choke housing counter clockwise until the choke is just closed. Do not tighten the choke plate any farther by turning the housing past where the choke closes, or it will not come off soon enough.

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