At idle speed, the throttle plate is slightly advanced from the completely closed position to expose about one-half of the idle discharge prot to engine manifold vacuum (suction). This suction is transmitted to the idle jet through a passage connecting the idle discharge port (priming plug) with the idle jet.
Fuel for idle operation 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 restricted drilling at the bottom of the idle fuel pickup passage. From here it is metered through the idle jet calibration before entering the vacuum passage leading to the idle discharge port.
As it leaves the idle jet the fuel is mixed with a variable amount of air admitted from behind the venturi. This fuel-air mixture then passes through the idle vacuum passage and is discharged into the air stream through the idle discharge port. Turning the idle adjusting needle IN (clockwise) increases the suction on the idle jet and reduces the amount of idle air resulting in a richer mixture. Turning the idle needle OUT (counter-clockwise) reduces the suction on the idle jet and increases the amount of air admitted for idle, resulting in a leaner mixure.
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 restricted drilling at the bottom of the idle fuel pickup 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 an air bleed which originates back of (or from behind) the venturi. The idle air bleed controls the air to be mixed with fuel for the idle fuel-air mixture.
This system is referred to as the two hole idle system since the fuel-air mixture is discharged into the air stream through one or both of the two calibrated holes located in the throttle body bore. The idle adjusting needle, located in the throttle body at the upper idle discharge hole, controls the amount of idle fuel-air mixture discharged into the air stream.
Turning the idle adjusting needle IN (clockwise) results in a leaner mixture since less of the fuel-air mixture is discharged into the air stream through the idle discharge hole. Turning the idle adjusting needle OUT (counter-clockwise) results in a richer idle fuel-air mixture since more of the fuel-air mixture is discharged into the air stream through the idle discharge hole.
Note that in this type of idle system, the idle adjusting needle controls the amount of the idle fuel-air mixture discharged into the air stream.