How an Mechanical automatic carburettor choke works

A mechanical automatic carburettor choke works when the engine is cold, press the accelerator pedal once before switching on the ignition.

This operates a mechanism by which a coil spring in the choke housing can hold the choke flap closed, and the throttle butterfly is opened slightly to provide a fast idle when the engine is first started.

The spring in the choke housing is controlled either by water temperature, or by hot air from a hot-box' attached to the exhaust manifold, or an electric element.

As the engine warms, the heat of the water, air or element, expands the coil spring and the choke flap open.

The choke flap is also controlled by a piston or diaphragm inside the choke housing. At low engine speeds, when inlet manifold depression is high, the piston or diaphragm opens the choke flap against the pressure of the coil spring.

 If the engine load is increased the manifold depression (suction) is reduced and the piston or diaphragm rises, allowing the coil spring to close the choke flap.

Possible problems Dirt or carbon inside the choke housing may jam the mechanism, so keeping the choke closed.