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    GM Corvette 1963-1982 Repair Guide

    Rochester 4MC, 4MV, M4MC, and E4ME Carburetors

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    The Rochester Quadrajet® carburetor is a two stage, four-barrel downdraft carburetor. The designation MC or MV refers to the type of choke system the carburetor is designed for. The MV model is equipped with a manifold mounted thermostatic choke coil. The MC model has a choke housing and coil mounted on the side of the float bowl.

    The primary side of the carburetor is equipped with 1 3 / 8 in. diameter bores and a triple venturi with plain tube nozzles. During off idle and part throttle operation, the fuel is metered through tapered metering rods operating in specially designed jets positioned by a manifold vacuum responsive piston.

    The secondary side of the carburetor contains two 2 1 / 4 in. bores. An air valve is used on the secondary side for metering control and supplements the primary bores.

    The secondary valve operates tapered metering rods which regulate the fuel in constant proportion to the air being supplied.

    E4ME carburetors are used only on 1980-81 engines equipped with computer controlled emissions systems.

    E4ME carburetors should be serviced only by professional service technicians.

    ADJUSTMENTS - Accelerator Pump



    See Figure 1

    Click image to see an enlarged view

    Fig. Fig. 1: Quadrajet accelerator pump adjustment-except E4ME carburetors

    1. Close the primary throttle valves by backing out the slow idle screw and making sure that the fast idle cam follower is off the steps of the fast idle cam.
    2.  
    3. Bend the secondary throttle closing tang away from the primary throttle lever.
    4.  
    5. With the pump in the appropriate hole in the pump lever, measure from the top of the choke valve wall to the top of the pump stem.
    6.  
    7. To adjust, bend the pump lever while supporting it with a screwdriver.
    8.  
    9. After adjusting, readjust the secondary throttle tang and the slow idle screw.
    10.  

    Idle Vent

    Remove the idle vent valve cover from the air horn, if so equipped. Close the vent valve and open the primary throttle until the vent valve arm touches the bi-metallic strip next to the valve. Measure the distance between the top of the choke valve wall and the top of the pump stem. Bend to adjust.

    Float Level See Figure 2

    Click image to see an enlarged view

    Fig. Fig. 2: Quadrajet float level adjustment

    Remove the top cover and gasket, and use an adjustable T-scale to measure the distance from the top of the float bowl gasket surface to the top of the float at a point 3 / 16 in. back from the toe of the float. While gauging the clearance, push down on the float retainer and press lightly on the float tab to seat the float needle. Bend the float tang to specifications.

    Fast Idle

    Close the primary throttles and position the cam follower above the high step of the fast-idle cam. Turn the fast-idle screw clockwise until it touches the lever then turn it down three full turns. Readjust the fast idle rpm with the engine cold and running (choke closed).

    Choke Rod See Figure 3

    Click image to see an enlarged view

    Fig. Fig. 3: Quadrajet choke rod adjustment

    This procedure applies only to models having the choke thermostat mounted on the intake manifold.

    Position the cam follower on the second step of the fast-idle cam, touching the high step. Close the choke valve and gauge the clearance between the lower edge of the valve and the body. Bend choke rod to obtain the specified clearance.

    Choke Coil Lever See Figure 4

    Click image to see an enlarged view

    Fig. Fig. 4: Quadrajet choke coil lever adjustment

    Refer to the accompanying illustration to perform this adjustment.

    Air Valve Rod See Figure 5

    Click image to see an enlarged view

    Fig. Fig. 5: Quadrajet air valve rod adjustment

    Refer to the accompanying illustration to perform this adjustment.

    Front Vacuum Break See Figures 6 and 7

    Click image to see an enlarged view

    Fig. Fig. 6: Quadrajet front vacuum break adjustment-early models without an adjusting screw



    Click image to see an enlarged view

    Fig. Fig. 7: Quadrajet front vacuum break adjustment-late models with an adjusting screw

    Refer to the accompanying illustrations to perform this adjustment.

    Choke Unloader

    Close the choke valve and secure it with a rubber band hooked to the vacuum break lever. Open the primary throttles all the way. Then measure the distance between the air horn and edge of the choke butterfly. On models up to and including 1976, use the bottom side of the butterfly for this measurement; on 1977 and later models, use the top side. Bend the fast idle lever tang to achieve the proper opening of the choke.

    Air Valve Spring See Figure 8

    Click image to see an enlarged view

    Fig. Fig. 8: Quadrajet air valve spring adjustment

    Loosening and tightening the locking screw to adjust the air valve spring requires a hex wrench on 1977 and later model carburetors.

    Remove all spring tension by loosening the locking screw and backing out the adjusting screw. Close the air valve and turn the adjusting screw in until the torsion spring touches the pin on the shaft, and then turn it the additional number of turns specified. Secure the locking screw.

    DISASSEMBLY & ASSEMBLY



    See Figures 9, 10 and 11

    Click image to see an enlarged view

    Fig. Fig. 9: Installation of the lower choke rod lever on a Quadrajet carburetor



    Click image to see an enlarged view

    Fig. Fig. 10: Correct positioning of the float needle clip on a Quadrajet carburetor-DO NOT put the clip into either of the float bracket holes



    Click image to see an enlarged view

    Fig. Fig. 11: You must follow the correct sequence when tightening the Quadrajet air horn screws

    Due to the many minor design changes in the Quadrajet®, some steps may not apply to your carburetor. E4ME carburetors should be serviced only by qualified, professional technicians.


    CAUTION
    NEVER immerse any plastic or rubber carburetor part in carburetor cleaner. Also, use extreme caution when handling the carburetor to prevent damaging the throttle shaft or valves.

    1. Remove the solenoid and bracket from the float bowl, if so equipped. Disconnect the upper choke rod and lever from the choke shaft (one screw). Rotate the lever to remove it from the rod. Remove the choke rod by holding the lower lever outward and twisting the rod counter-clockwise.
    2.  
    3. Remove the secondary metering rods and the hanger by removing the small screw from the hanger. Using a narrow drift punch, tap the accelerator pump lever pivot pin inward, just enough to release the lever. Grasp the lever and pump rod (together) and rotate to remove from the throttle linkage.
    4.  
    5. Remove the air horn-to-float bowl screws (nine).
    6.  

    Be sure to remove the two screws located within the primary venturi area. You may not notice these screws at first glance.

    Remove the air horn assembly by lifting it straight up, off of the float bowl.


    CAUTION
    Use care to avoid damaging the small brass tubes protruding from the air horn-these tubes are permanently pressed into the air horn.

    1. Remove the vacuum line from the vacuum break unit. Remove the vacuum break assembly and disconnect the rod from the secondary air valve lever.
    2.  


    CAUTION
    Further disassembly of the air horn is not required. DO NOT remove any plugs which are located on the air horn.

    1. Remove the air horn gasket from the float bowl. Use care to withdraw the gasket from beneath the primary metering rod hanger. Remove the accelerator pump plunger from the pump well.
    2.  
    3. Remove the power piston and metering rods by pushing downward on the piston and allowing the assembly to snap back (upward). This may have to be repeated several times to free the plastic power piston retainer ring.
    4.  


    CAUTION
    Do not pry or pull the power piston from the piston well.

    1. Remove the power piston spring from the well.
    2.  
    3. Remove the plastic filler block from the float bowl. Remove the float, float retainer, and needle from the float bowl. Using a suitable prytool, remove the needle seat and gasket.
    4.  
    5. Remove the plastic aneroid cavity insert from the float bowl. If desired, unscrew and remove the primary main metering jets from the float bowl.
    6.  


    CAUTION
    DO NOT attempt to remove the secondary main metering jets from the float bowl, as these jets are permanently fixed to the bowl.

    1. Remove the accelerator pump discharge ball and retainer (appears as a screw next to the pump well). Remove the secondary air baffle, if it is to be replaced.
    2.  

    On models using an intake manifold mounted choke thermostat, disassembly of the choke mechanism is not required to clean the carburetor. On models using a hot air carburetor mounted choke assembly, it is not necessary to disassemble the choke mechanism if you plan to spray carburetor cleaner on the carburetor passages only-if you plan to immerse the carburetor, follow the next two steps to remove the choke assembly from the float bowl.

    1. Remove the choke thermostat cover from the choke housing (three screws). Remove the choke housing retaining screw from inside the choke housing. Remove the complete choke housing assembly from the fuel bowl by sliding it outward.
    2.  
    3. Remove the secondary lock-out lever from the float bowl. Remove the lower choke lever from inside the float bowl by inverting the float bowl.
    4.  


    CAUTION
    Do not immerse the choke housing assembly in carburetor cleaner.

    1. Remove the fuel inlet nut, gasket, and filter. Remove the throttle body-to-fuel bowl screws then remove the throttle body assembly and the gasket.
    2.  

    Assembly is the reverse of the previous steps. Note the following points during assembly:

    1. Install the lower choke rod lever as shown in the accompanying illustration.
    2.  
    3. When installing the float and needle, DO NOT put the needle clip into either of the float bracket holes. The clip must be installed as shown in the accompanying illustration.
    4.  
    5. Install the plastic filler block BEFORE the metering rods and hanger. When installing the primary metering rods, make sure the rod tips are positioned in the primary jets, then lower the power piston. To seat the plastic power piston retainer, evenly push it into the top of the piston bore with a small prytool.
    6.  
    7. When installing the air horn assembly, be sure to follow the accompanying torque sequence diagram for the air horn screws.
    8.  
    9. When installing the accelerator pump lever and rod, engage the lower end of the rod into the throttle linkage, and align the lever pivot hole with the pivot pin. Push the pin through by carefully prying it with a suitable prytool (between the pin and the choke plate wall).
    10.  

    CEC VALVE MAINTENANCE - 1971 Quadrajet Only



    See Figure 12

    Click image to see an enlarged view

    Fig. Fig. 12: Location of the Quadrajet CEC valve

    The Rochester 4MV Quadrajet has a number of revisions for 1971 which include: calibration changes, greater capacity accelerator pump, increase in the size of the vacuum break diaphragm restriction to 0.020 in., a beefed-up, choke-closing assist spring, removal of the wide open kick lever from the choke unloader mechanism and its replacement with a tang on the fast-idle lever that contacts the fast-idle cam and forces the choke valve to admit more air under flooded engine conditions, and the adaption of a Combination Emission Control (CEC) valve. This valve increases the idle speed during high gear overrun and helps to control normally unburned hydrocarbons. The mixture is set at the factory and the idle mixture screws are capped to prevent adjustment in the field.

    When disassembling the carburetor, remove the CEC valve from the carburetor, but leave the valve bracket attached to the carburetor. Do not immerse the CEC valve in carburetor cleaner. Disassembly practices differ in the 1971 4MV due to calibration changes. Of primary importance is the revised procedure for removing the power piston. Do not use pliers, but press the piston down and release it. Do not remove the idle mixture screws.

    If it should be necessary to replace the idle-mixture adjustment screws, carefully bottom the old screws and count the turns so that the replacement screws will retain the same mixture.

    If the throttle body is replaced, it will be necessary to adjust the idle mixture screws. Before making the adjustment, read and follow the instructions on the inner fender tune-up sticker on each 1971 and later Corvette. Lightly bottom the mixture screws and back out four turns. Set the idle speed adjusting screw to obtain the specified initial idle speed, then make equal adjustments to the mixture screws until the required carbon monoxide level is reached in the exhaust gas. Adjust the idle speed screw until the final idle setting is achieved. Install the idle mixture screws limiter caps and reconnect the distributor vacuum hose and the fuel tank vapor hose.

    An alternate method is available if access to exhaust gas analyzing equipment is not practical. Follow the same procedure until the mixture adjustment stage. Adjust the mixture screws equally leaner until the final idle speed is achieved. Install the limiter cap, vacuum hose, and tank vapor hose.

     
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