Idle Circuit : During Idling while the throttle valve is almost closed the vacuum is very high at Venturi area and no air-fuel mixture is drawn through the main circuit, as the jet needle is not lifted from the seat. At idle, the throttle valve is pushed most of the way down by a spring, almost completely closing off the main bore. To keep the idle conditions stable, a slow speed circuit is devised and controlled through the air-screw where the air-fuel mixture is regulated. Because of suction, the air-fuel mixture is delivered in atomized form through the slow jet opening.

Figure : Carburettor in Idle Condition