The Quadrajet carburetor idle system is located on the primary side (fuel inlet side) of the carburetor to supply the correct air/fuel mixture during idle and off-idle operation.

The idle system is used during this period because air flow through the carburetor venturi is not great enough to obtain efficient metering from the main discharge nozzles.  The idle system is only used in the two primary bores of the carburetor.  Each bore has a separate and independent idle circuit (Figure 11).

The idle system consists of a calibrated idle tube, air bleeds, idle channel restriction, idle mixture discharge hole, and idle mixture screws or needles (one for each bore).

The idle system operates as follows:

During curb idle (warm engine), the primary throttle valve is held slightly open by the idle speed screw or solenoid plunger.  The small amount of air passing between the throttle valve and bore is regulated by adjusting the position of the speed screw or solenoid plunger to obtain the desired idle speed.  Since the engine requires very little air for idle and low speed, fuel is added to the air to produce a combustible mixture by the direct application of vacuum (low pressure) from the intake manifold to the idle discharge hole below the throttle valve.  The idle discharge hole is in a very low pressure area and the fuel in the fuel chamber is vented to atmosphere (high pressure through the air cleaner).  This causes fuel to flow from the fuel chamber down through the main metering jet into the main fuel well.  It is picked up in the main well by the idle tubes (one for each bore) which extend into the fuel well. The fuel is metered at the lower tip of the submerged idle tube and passes up through the tube.  At this point, the fuel is mixed with air bleed.  The air bleed size is controlled by either a drilled hole or a brass inserts depending upon carburetor application.

          NOTICE:  No attempt should be made to install a brass insert in those applications that use a drilled hole for the top idle air bleed.

Then the fuel mixture crosses over to the idle down channel where it is mixed with air at the side idle bleed located just above the idle channel restriction.  The mixture continues down through the calibrated idle channel restriction past the lower idle air bleed off-idle discharge port where it is further mixed with air.  The air/fuel mixture moves down to the adjustable mixture screw discharge hole where it enters the carburetor bore and blends with the air passing by the slightly open throttle valve.  The combustible air/fuel mixture then passes through the intake manifold to the engine cylinders.

The idle mixture screws are adjustable to blend the correct amount of air/fuel mixture from the idle system to the engine at idle.  Turning the mixture screws inward (clockwise) decreases idle mixture discharge (gives a leaner mixture) and turning the mixture screws outward (counter-clockwise) increases idle mixture discharge (gives a richer mixture).