At idle speed the throttle plate is advanced slightly to expose the upper idle discharge hole to engine manifold vacuum (suction). This suction is transmitted to the idle jet through a passage connecting the idle discharge holes with the idle jet. Fuel for idle is supplied through the main jet to a well directly below the main discharge jet. The fuel for idle flows from this well through a restriction at the bottom of the idle fuel passage. From here it is metered through the idle jet calibration before entering the vacuum passage leading to the idle discharge holes. As the fuel leaves the idle jet, it is mixed with air admitted through the idle air bleed which originates back of (or behind) the venturi. This fuel-air mixture in the idle passage is discharged into the air stream through the idle dischrage holes. The idle adjusting needle controls the suction on the idle jet as well as the amount of air taken in through the idle air bleed. Turning the idle adjusting needle IN (clockwise) reduces the volume of air admitted to the idle system and increases the suction on the idle jet. Hence more fuel flows through the idle system. Turning the idle adjusting needle OUT (counter-clockwise) increases the volume of air admitted to the idle system and decreases the suction on the idle jet which results in less fuel flowing through the idle system to give a leaner mixture. As the throttle plate is advanced slightly, the second idle discharge hole is uncovered to increase the amount of fuel-air mixture discharged through the idle system.