See Figure 1
This system is used only on carbureted vehicles.
The jet air control valve system consists of a jet air control valve, which is an integral part of the carburetor, and a thermo-valve which is controlled by coolant temperature. Its purpose is to help decrease hydrocarbons and carbon monoxide during engine warm-up while the choke is operating.
Carburetor vacuum opens the valve thereby allowing air to flow into the jet air passage preventing an overly rich air-fuel mixture. There is also a thermo-valve in the system. The function of the thermo-valve is to stop jet valve operation when the coolant temperature is above or below a pre-set value.