Auxiliary Air Valve (Idle Speed): Description and Operation

Fig. 22 Auxiliary air regulator:

Fig. 23 Auxiliary air regulator internal view:

The auxiliary air regulator prevents stalling at idle during engine warm-up by supplying additional air to the engine, Fig. 22.
Since any engine is less efficient when cold, some provisions like a carburetor "fast-idle" are necessary. The auxiliary air regulator controls the amount of air bypassing the closed throttle valve. It is calibrated so there is little difference in the idle speed of a cold or warm engine.
When the engine is cold, the gate valve is open. The heating coil is connected to the fuel pump and the control pressure regulator circuit so that as the engine warms up, the bi-metallic strip is heated. The deflection of the bi-metallic strip rotates the gate valve, gradually cutting off the passage for additional air as the engine warms up, Fig. 23.