At idle speed. the throttle plate is slightly advanced from the completely closed position to expose about one-half the area of the idle discharge port to engine manifold vacuum (suction). This suction is transmitted to the idle jet through a passage connecting the idle dischrage port with the idle jet. Fuel for idle operation is supplied through the main jet to th emetering well through the idle feed holes and idle fuel restriction into the idle well. From here the fuel is metered through the idle jet calibration before entering the vaccum passage leading to the idle discharge port. As the fuel leaves the idle jet, it is mixed with a variable amount of air admitted from behind the venturi through the idle air passage. This fuel air mixture then passes through the idle vacuum passage and is discharged into the air stream at the idle discharge port. Turning the idle adjusting needle IN (clockwise) increases the suction on the idle jet and causes more fuel to flow through the idle jet calibration. Turning the idle adjusting needle OUT (counter-clockwise) reduces the suction on the idle jet and causes less fuel to flow through the jet.