Screenshot 2025-06-05 at 2.35.35 PM.png

At idle speed the throttle plate in 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. The fuel for idle flows out of this well through a restricted drilling at the bottom of the idle fuel pick-up passage. From here the fuel 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 that originates back of (or from behind) the venturi. The position of the idle adjusting needle in this passage controls the suction on the idle jet and thereby the idle fuel-air mixture.

Turning the idle adjusting needle IN (clockwise) results in a greater suction on the idle jet with a smaller amoung of air admitted to give a richer mixture. Turning the needle OUT (counterclockwise) increases the amount of idle air admitted and reduces the suction on the idle jet resulting in a leaner mixture. This idle fuel air mixture is then discharged through the idle discharge holes into the air stream.