On some 4MV Quadrajet applications, a carburetor mounted Combination Emission Control (C.E.C.) valve is added to help reduce exhaust emissions (Figure 38).  A vacuum tube in the float bowl is connected by hose to the C.E.C. valve which, when energized by a transmission signal, controls distributor vacuum spark advance by providing spark vacuum advance during transmission high-gear operation (and in reverse on turbo-hydramatic transmission applications) and, when de-energized, retarded spark timing during operation of the transmission in lower gears and at idle.

The C.E.C. valve, when energized through the transmission, also acts as a throttle stop by increasing engine idle speed during high-gear operation (and in reverse on turbo-hydramatic transmission applications) to add more air as an aid in controlling over-run hydrocarbons (unburned fuel) during deceleration.  Normal idle speed setting is made with the idle stop screw.  Idle speed settings should be made following vehicle manufacturer’s specification (noted on the emission label in the engine compartment).

The C.E.C. valve may be identified from the idle stop solenoid by two vacuum tubes for distributor vacuum advance, located at the end of the valve, and by the following precautionary label affixed to the valve:

        “CAUTION: Never use to set idle. See Service Manual for adjustments.”

IDLE STOP SOLENOID

An electrically operated idle stop solenoid (Figure 39), mounted on a bracket on the carburetor float bowl, is added on some Quadrajet carburetors.  The idle stop solenoid controls the engine curb idle speed.  Curb idle speed setting is made by adjusting the plunger screw in the idle stop solenoid with the solenoid energized electrically.  The low idle speed is adjusted by turning the low idle speed screw on the carburetor bowl.  Curb idle and low idle speed settings should be made using information from the emission information label located in the engine compartment (1968 and later vehicles).

Some air conditioned vehicles with the Quadrajet carburetor use an A/C idle speed solenoid mounted on the carburetor float bowl in place of the idle stop solenoid.  The A/C idle speed solenoid is electrically energized only when the air conditioning compressor is engaged (air conditioning “On”) to maintain proper idle speed when the air conditioning is in operation. Normal curb idle speed (air conditioning “Off”) is made using the idle speed screw on the carburetor bowl.

Other Quadrajet models use a throttle closing dashpot unit mounted in place of the idle stop solenoid.  The purpose of the closing dashpot is to retard throttle closing during deceleration to prevent backfire in the exhaust system on certain applications.

THROTTLE LEVER ACTUATOR

Some heavy duty truck Quadrajet applications use a throttle return control (TRC) system.  The system consists of a throttle return control valve, mounted separately from the carburetor, and a throttle lever actuator mounted on the carburetor float bowl on the throttle lever side (Figure 40).

The throttle return control valve is an on-off valve that senses engine manifold vacuum and opens above a preset high vacuum level.  The valve, when open, allows a vacuum signal to be applied to the throttle lever actuator.

The TRC system operates as follows:

During prolonged periods of deceleration, when a preset high vacuum level is reached, the TRC valve opens and allows a vacuum signal, through the vacuum inlet tube, to be applied to the throttle lever actuator diaphragm.  This vacuum level is sufficient to cause the actuator diaphragm to move downward against the calibrated return spring force and the plunger stem moves outward.  At this point, the plunger contacts a tang on the throttle lever to open the primary throttle valves slightly to reduce exhaust emissions.  The TRC valve remains open and vacuum is applied as long as the preset vacuum level at the TRC valve is exceeded.  The throttle lever actuator plunger may be adjusted to provide the required throttle opening during deceleration conditions.

IDLE LOAD COMPENSATOR (ILC)

Some late model carburetors have an Idle Load Compensator rather than an idle speed solenoid.  The compensator (Figure 41) is mounted on the float bowl and uses manifold vacuum to sense changes in engine load and compensates by adjusting curb idle speed. The idle load compensator uses a spring-loaded vacuum sensitive diaphragm unit with a plunger that either extends (as manifold vacuum decreases) or retracts (as manifold vacuum increases) to open or close the throttle valves to control idle speed.  The ILC is only serviced as a complete assembly.

         NOTICE: The Idle Load Compensator (ILC) is adjusted at the factory.  No attempt should be made to adjust the Idle Load Compensator unless, in diagnosis, curb idle speed is not to factory specifications. If adjustment is necessary, refer to Service Manual for proper procedures.