The Quadrajet carburetor has an idle system on the primary side (fuel inlet side) of the carburetor to supply the correct air/fuel mixture ratios 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 system. They consist of idle tubes, idle passages, idle air bleeds, ilde channel restrictions, idle mixture adjustment needles and idle discharge holes.
During curb idle the throttle valve is held slightly open by the idle speed adjusting screw. The small amount of air which passes betwen the primary throttle valve and bore is regulated by this screw to give the engine the desired idle speed. Since the engien requires very little air for idle and low speeds, fuel is added to the air to produce a combustible mixture by the direct application of vacuum (low pressure) from the engine manifold to the idle discharge hole below the throttle valve. With the idle discharge holes in a very low pressure area and the fuel in the float bowl vented to atmosphere (high pressure) the idle system operates as follows:
Fuel flows from the float bowl down through the main metering jets into the main fuel wells. It is picked up in the main wells by the two idle tubes (one for each primary bore) which extend into the wells. The fuel is metered at the lower tip of the idle tube and passes up through the tubes. On some models, where needed, the fuel is mixed with air at the top of each idle tube through an idle air bleed. The fuel mixture crosses over to the idle down channels 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 restrictions past the lower idle air bleeds and off-idle discharge ports where it is further mixed with air.
The air/fuel mixture moves down to the adjustable idle mixture needle discharge holes where it enters the carburetor bores and blends with the air passing the slightly open throttle valves. The combustible air/fuel mixture then passes through the intake manifold to the engine cylinders.
The idle mixture needles are adjustable, to blend the correct amount of fuel mixture from the idle system with the air entering the engine at idle. Turning the idle mixture needle inward (clockwise) decreases the idle fuel discharge (gives a leaner mixture) and turning the mixture screws outward (counter-clockwise) enriches the engine idle mixture.
Some carburetor models have a fixed idle air bypass system. This consists of air channels which lead from the top of each carburetor bore in the air horn to a point below each primary throttle valve. At normal idle, extra air passes through these channels supplementing the air passing by the slightly opened throttle valves. The purpose of the idle air bypass system is to allow reduction in the amount of air going past the throttle valves so they can be nearly closed at idle. This reduces the amount of air flowing through the carburetor venturi to prevent the main fuel nozzles from feeding during idle operation. The venturi system is very sensitive to air flow and on some applications where larger amounts of idle air are needed to maintain idle speed, the fixed idle air by-pass system is used.
Another feature added to emission carburetors is an adjustable off-idle air bleed system. A separate air channel is added in the air horn which leads from the top of the air horn to the idle mixture cross channel. An adjustment screw with a tapered head is mounted at the top of the channel and is used to control the amount of air bleeding into the idle system. The off-idle air bleed is adjusted at the factory to maintain very accurate off-idle air fuel mixture ratios. It is adjusted during carburetor flow test and no attempt should be made to readjust in the field. A triangular spring clamp forced over the vent tube covers the screw to protect the adjustment from being tampered with and should not be removed. All service air horns have this screw preset at the factory.
OFF IDLE OPERATION
As the primary throttle valves are opened from curb idle to increase engine speed, additional fuel is needed to combine with the extra air entering the engine. This is accomplished by the slotted off-idle discharge ports. As the primary throttle valves open, they pass by the off-idle ports, gradually exposing them to high engine vacuum below the throttle valves. The additional fuel added from the off-idle ports mixes with the increasing air flow past the opening throttle valves to meet increased engine air and fuel demands.
Further opening of the throttle valves increases the air velocity through the carburetor venturi sufficiently to cause low pressure at the lower idle air bleeds. As a resulf, fuel begins to discharge from the lower idle air bleed holes and continues to do so throughout operation of the part throttle to wide open throttle ranges, supplementing the main discharge nozzle delivery.
The idle needle holes and off-idle discharge ports continue to supply sufficient fuel for engine requirements until air velocity is high enough in the venturi area to obtain efficient fuel flow from the main metering system.
In operation, the idle vent valve is closed except when the throttle valves are in the idle position. When the throttles are closed, a wire tang on the pump lever pushes upward on the spring steel vent valve arm and opens the vent valve. Thus fuel vapors are allowed to vent externally, thereby preventing them from entering the carburetor bores and being drawn into the engine. This prevents rough engine idle and excessively long, hot engine starting. When the throttle valves are open to the off-idle and part throttle position, the idle vent valve closes, returning the carburetor to internally balanced venting.
A temperature controlled idle vent valve is used on some models. In place of the standard vent valve, a heat sensitive bi-metal strip is used as the valve holder. This is mounted beneath the idle vent valve arm.
The bi-metal strip holes the vent valve on its seat (closed) at temperatures below 75*. When underhood temperatures are above 75* to 85* the bi-metal strip bends upward moving the vent valve off its seat. This lets fuel vapors, caused during hot engine operation, escape from the float chamber. This results in improved hot engine idle and hot starting. At temperatures below 75*, the vent valve remains closed and retains fuel vapors internally to supply extra fuel for good cold engine starting.
During hot engine operation, when the thermostat vent valve is open, it is necessary to close the valve except at idle to maintain an internally balanced carburetor. This is accomplished through the spring steel vent valve arm which operates off the wire lever on the end of the pump lever. As the throttle valves are opened from the idle position, the vent arm exerts pressure on the bi-metal strip and forces the valve closed.
The thermostatic vent valve is adjustable to make sure it closes at the proper time during throttle valve opening from the idle position.
Hesitation off of idle can be caused by electrical, accelerator pump and a little known idle passage way. This video is about that passage way and how it works.