Automatic
Choke
A device that automatically reduces air flow into a carburetor when the engine is cold to make the air/fuel mixture richer and help the engine start faster. To reduce the air flow to the carburetor, the automatic choke uses a butterfly valve ? a shaft with a semicircular wing on each side ? which blocks the air flow through the carburetor?s throat when the shaft is turned. This valve is known as the choke plate.
Automatic chokes are of two types: integral (built directly onto the carburetor) or remote (attached to the exhaust manifold). A bimetallic coil or spring (a spring made from two dissimilar metals) is used to close the choke plate. A vacuum opens the plate. When the engine is cold, the spring holds the choke plate closed. When the engine is started, the closed choke creates a vacuum in the carburetor air horn which sucks a large amount of fuel out of the main discharge tube. As the engine and spring warm, the spring uncoils allowing the to open the choke plate, resulting in a leaner mixture.
Four main methods are used to control the warming of the choke thermostatic spring:
1. An electric coil may be used to heat the thermostatic spring. The heating coil is switched on with the ignition switch. Some systems use a control unit that prevents power from reaching the electric coil until the engine compartment reaches a desired temperature.
2. Engine coolant may be circulated via a passage in the thermostat housing to heat the spring.
3. In well-type heated systems, the thermostatic spring is mounted on top of the exhaust manifold. As the engine and manifold warms, the spring uncoils and the choke open.
4. Exhaust heat may be brought to the choke through the means of a heat tube. The heat tube passes through the exhaust manifold, so as it takes in fresh air via the choke stove, it picks up heat from the exhaust without sending any actual exhaust fumes to the choke mechanism.
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