As the primary throttle valve is 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 port.  As the primary throttle valve opens, it passes by the slotted off-idle discharge port, gradually exposing it to high manifold vacuum.  The mixture added from the off-idle port mixes with the increasing air flow past the opening throttle valve to maintain the required air/fuel mixture of the engine.

Further opening of the throttle valve increases the air velocity through the carburetor venturi sufficiently to cause low pressure at the lower idle air bleed.  As a result, fuel begins to discharge from the lower idle air bleed hole and continues throughout operation of the part throttle to wide-open ranges, thereby supplementing main discharge nozzle delivery.

The idle mixture screw discharge hole and off idle discharge port 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.

The idle system functions in a similar manner in each carburetor bore.

The secondary throttle valves remain closed during idle conditions and on some Quadrajet applications they are coated with a special graphite material which is applied at the factory to effectively seal the secondary throttle valves in the bores for minimum air flow at idle to prevent possible rough idle.

On many exhaust emission control carburetor applications, the idle mixture screw discharge holes have been reduced in size.  This was done to prevent a too rich idle adjustment in the field should the idle mixture screws be turned out too far beyond normal idle mixture requirements.  Also, starting in 1971, idle screw limiter caps were added to emission control carburetors to discourage adjustment of the screws in the field. On later models, the idle mixture screws are sealed to prevent readjustment from the factory setting in the field.

ADJUSTABLE AIR BLEED

Another feature added to some emission carburetors is an adjustable off-idle air bleed system (Figure 11).  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 it should not be removed.  All service air horns have this screw preset at the factory.

FIXED IDLE AIR BY-PASS

A fixed idle air by-pass system is used on some applications which consist of air channels that lead from the top of each carburetor bore in the air horn to a point below each throttle valve.  At normal idle, extra air passes through these channels supplementing the air passing by the slightly open throttle valves.  The purpose of the idle air by-pass system is to reduce the amount of air going past the throttle valves so that they are nearly closed at idle.  This reduces the amount of air flowing through the carburetor venturi to prevent the main discharge nozzles from feeding fuel during idle operation.  The triple venturi system is very sensitive to air flow and where large amounts of idle air are needed to maintain idle speed, the fixed idle air by-pass system is used.

PURGE PORTING

To meet evaporative emission standards on late model Quadrajet applications, fuel vapors from the carburetor float bowl and fuel tank are collected in a vapor canister and not vented to atmosphere.  On many of these carburetor models, vacuum ports are located in the throttle body for canister purge.  The purge ports lead through passages to a tube in the throttle body which connects by a hose to the vapor canister.

The purge ports may consist of a constant (fixed) canister purge and a separate timed canister purge, or a timed canister purge only (Figure 12).

The purge system operates as follows:

CONSTANT CANISTER PURGE

The constant canister purge port operates during idle operation of the engine.  It is located below the throttle valves to provide a constant purge of the canister whenever the engine is running.  As the throttle valves are opened beyond the idle position, additional purge of the canister is provided by each of the timed purge ports.

TIMED CANISTER PURGE

The timed canister purge ports operated during off-idle, part throttle, and wide-open throttle operation.  They are located above the throttle valve in each bore next to the off-idle discharge port.  The timed purged ports supplement the constant canister purge port (where used) to provide a larger purge capacity for the vapor canister and to prevent over-rich mixtures from being added to the carburetor metering at any time.

On some applications (see Figure 12), the constant purge port is used as the vacuum source to the Early Fuel Evaporation (E.F.E.) Valve located in the exhaust manifold.  On these models, purging of the canister is accomplished through the Positive Crankcase Ventilation (P.C.V.) system.

EXHAUST GAS RECIRCULATION

An Exhaust Gas Recirculation (E.G.R.) system, consisting of an E.G.R. Valve, separate back-pressure transducer valve (where used), vacuum hoses, and vacuum supply ports in the Quadrajet carburetor, is used on many vehicle applications to meet exhaust emission requirements. The E.G.R. valve is operated by a vacuum signal taken from the carburetor.  The vacuum signal, dependent upon application, is taken from one or two punched ports located in the carburetor bore just above the throttle valves.  Thus, E.G.R. valve operation is “timedâ€� for metering exhaust gases to the intake manifold dependent upon location of the ports in the carburetor primary bore and by the degree of throttle valve opening (See Figure 12).

It is important that the E.G.R. vacuum signal port(s) not be exposed to manifold vacuum during engine idle and deceleration to keep the E.G.R. valve closed.  This prevents rough idle which can be caused by excessive exhaust gas contamination in the air/fuel mixtures.

TRANSMISSION CONVERTER CLUTCH

Some applications have a port located above the throttle valve that is used to supply a timed vacuum source for the automatic transmission converter clutch (see car division service manual for description of this transmission feature.)

HOT IDLE COMPENSATOR

The Hot Idle Compensator, when used on 4MV-4MC-M4MC Quadrajet carburetors (see Figure 11), is located in a chamber at the rear of the carburetor float bowl adjacent to the secondary bores.  Its purpose is to offset the enriching effects caused by excessive fuel vapors during hot engine operation.

The compensator consists of a thermostatically-controlled valve, a heat sensitive bi-metal strip, and a valve holder and bracket.  The compensator valve assembly is held in place by a dust cover over the valve chamber.  A seal is used between the compensator valve and the float bowl casting.  The valve closes off an air channel leading from a hole in the top of the air horn, just beneath the air cleaner, to a point below the secondary throttle valves.

Normally, the compensator valve is held closed by tension of the bi-metal strip.  During extreme hot engine operation, excessive fuel vapors entering the engine manifold cause richer than normally required mixtures, resulting in rough engine idle and stalling.  At a predetermined temperature, when extra air is needed to offset the enriching effects of these fuel vapors, the bi-metal strip bends and unseats the compensator valve.  This uncovers the air channel leading from the valve chamber to the point below the throttle valves. This allows enough air to be drawn into the engine manifold to offset the richer mixtures and maintain a smooth engine idle.  When the engine cools and the extra air is not needed, the bi-metal strip relaxes, closes the valve, and operation returns to normal mixtures.

For proper idle adjustment when the engine is hot, the compensator valve must be closed.  To check this, a finger may be held over the compensator air inlet channel located on top on the air horn.  If no drop in engine RPM is noted on a tachometer, the valve is closed.  If the valve is open, plug the hole or cool engine down to a point where the valve automatically closes for proper idle adjustment.

         NOTICE: Plug the compensator hole with a pencil or something that will be seen, as the plug must be removed before the air cleaner is installed.  Otherwise the compensator will not function if the plug is left in the hole.

On some applications, the air inlet to the hot idle compensator is located beneath the air valve in the secondary bores.  The air inlet in this location improves idle quality when the hot idle compensator valve is open.  The compensator valve can be checked for proper closing during idle adjustment by pushing inward on a spring-loaded plunger mounted in the idle compensator cover.  The idle adjustment procedure is the same as recommended previously.

On some Model 4MV carburetors, the Hot Idle Compensator is located on the primary side of the float bowl.  It is mounted in the bowl with a pin protruding through the air horn casting to facilitate closing the valve when the idle adjustment is made. The air inlet for the compensator is located in the cavity next to the choke rod in the bowl.

When the compensator opens, filtered air is drawn around the choke rod through a passage in the float bowl and throttle body and on into the intake manifold.  The air enters the manifold through the opening in between the primary bores.  This air mixes with the air/fuel mixture during hot engine operation.