How Does an Automatic Choke Carburetor Work?by Erik Arvidson
In older vehicles that use a carburetor rather than an electronic fuel injection system to deliver the mixture of air and fuel to the engine, it's the job of the choke valve to control the flow of air and ensure that the engine starts properly.
The function of the carburetor is to provide a rich fuel/air mixture to the engine. When there is too much fuel in the mixture, it's known as "rich," and when there is too much air, it's known as "lean." Modern electronic carburetors use feedback from sensors to adjust the fuel/air mixture to the level needed to operate the engine and minimize harmful emissions.
When an engine is started, the fuel/air mixture should be rich, because fuel vaporizes slowly when it's cool. The choke, which is situated at the top of the carburetor, will close off, or "choke" the air supply to the carburetor. As the engine warms up, it needs more air, so the choke plate shaft moves to the side to allow greater airflow.
Most vehicles use an automatic choke. Typically, a thermostat in the choke will sense increasing engine heat that is being transmitted to the choke housing, and cause a bimetal spring to relax and open the choke valve.