The idle system supplies air fuel mixture to operate the engine at idle & low speeds. Idle fuel enters the system through the main jet into the main. well. Some applications include an idle tube in the main well to meter the fuel. Other applications use a horizontal passage from the main well to the idle well and an idle channel restriction for metering purposes. The restriction is installed in several different locations.
In either type, air enters the idle air bleed and mixes with the fuel after the fuel flows through the idle tube or restriction. At idle the throttle valves are almost closed. This creates a high vacuum below the throttle valve with near atmospheric pressure above the valve. As the throttle valves open the transfer slot is progressively exposed to vacuum and the air fuel emulsion is also discharge from the transfer slot. The air fuel mixture from the idle passage and the transfer slot supplies the necessary fuel to operate the engine at lower speeds and loads.