AutoZone 1985 AMC Eagle 4.2L 2BL 6cyl | Repair Guides | Fuel System | Carbureted Fuel System | Rochester 4mc, 4mv And M4mc 4-bbl. Carburetors | AutoZone.com

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    Chevrolet Full-size Cars 1968-1978 Repair Guide

    Rochester 4MC, 4MV and M4MC 4-Bbl. Carburetors

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    Fig. Fig. 1 Rochester 4MV, 4MC, M4MC Specifications



    Click image to see an enlarged view

    Fig. Fig. 2 Rochester 4MV, 4MC, M4MC Specifications



    Click image to see an enlarged view

    Fig. Fig. 3 Air horn tightening sequence for the Rochester M4MC carburetor

    The Rochester Quadrajet carburetor is a two stage, 4-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 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 " 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 " bores. An air valve is used on the secondary side for metering control and supplements the primary bores.

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

    ADJUSTMENTS



    Accelerator Pump
    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, using the proper tool.
    8.  
    9. After adjusting, reading the secondary throttle tang and the slow idle screw.
    10.  

    Idle Vent

    This adjustment is not required on 1977-78 carburetors.

    After adjusting the accelerator pump rod as specified above, open the primary throttle valve enough to just close the idle vent. Measure from the top of the choke valve wall to the top of the pump plunger stem. If adjustment is necessary, bend the wire tang on the pump lever.

    Float Level

    With the air horn assembly upside down, measure the distance from the air horn gasket surface (gasket removed) to the top of the float at the toe. Measure at a point 3 / 16 " back from the top of the float on 1977-78 carburetors.

    Make sure the retaining pin is firmly held in place and that the tang of the float is firmly against the needle and seat assembly.

    Fast Idle
    1. Position the fast idle lever on the high step of the fast idle cam. Disconnect and plug the vacuum hose at the EGR valve.
    2.  
    3. Be sure that the choke is wide open and the engine warm.
    4.  
    5. Turn the fast idle screw to gain the proper fast idle rpm.
    6.  

    Choke Rod

    Position the cam follower on the second step of the fast idle cam, touching the high step. Close the choke valve directly on models up to 1976. On 1977 models, remove the choke thermostatic cover, and then hold the choke closed by pushing upward on the choke coil lever. Gauge the clearance between the lower edge of the choke valve and the carburetor body on models to 1975, and between the upper edge of the choke valve and the carburetor body on 1976 and 1977 models. Bend the choke rod to obtain the specified clearance. On 1977 models, install the choke thermostatic cover and adjust it to specification when the adjustment is complete. This adjustment requires sophisticated special tools on 1978 models, and so is not included here.

    Air Valve Dashpot


    Click image to see an enlarged view

    Fig. Fig. 4 Adjusting the air valve dashpot on the Rochester 4MV carburetor

    Set the vacuum break diaphragm. On 1977-78 models, this requires plugging the bleed purge hole on the back of the diaphragm with tape, and the use of an external vacuum source. Hold the air valve tightly closed on all models. Gauge the clearance between the dashpot rod and the end of the slot in the air valve lever. Bend the rod to adjust. Remove the tape from the bleed purge hole.

    Choke Coil Rod
    1968-76 MODELS
    1. Close the choke valve by rotating the choke coil lever counterclockwise.
    2.  
    3. Disconnect the thermostatic coil rod from the upper lever.
    4.  
    5. Push down on the rod until it contacts the bracket of the coil.
    6.  
    7. The rod must fit in the notch of the upper lever.
    8.  
    9. If it does not, it must be bent on the curved portion just below the upper lever.
    10.  

    Choke Coil Lever and Choke Thermostatic Coil
    1977-78 MODELS
    1. Remove the three mounting screws and retainers, and pull the thermostatic coil cover assembly off the choke housing and set it aside.
    2.  
    3. Place the fast idle cam follower on the high step of the cam and then push up on the thermostatic coil tang in the choke housing until the choke is closed.
    4.  
    5. Insert a 0.120" plug gauge or rod of that diameter into the hole in the housing located just below the lever. With the choke closed, the lever should just touch the gauge.
    6.  
    7. Adjust the choke rod by changing the angle of the bend it makes just below the choke itself, if necessary.
    8.  
    9. Then, install the coil cover back on the choke housing, making sure that the thermostatic coil engages the tang. Install the three retainers and screws, but do not tighten. With the fast idle cam follower still on the high step of the cam, rotate the cover assembly counterclockwise until the choke closes. Set all models 2 notches lean except 1977 models with manual transmission; set these three notches lean. Hold the position of the housing while tightening screws evenly.
    10.  

    Vacuum Break
    1968-76 MODELS
    1. Fully seat the vacuum break diaphragm using an outside vacuum source.
    2.  
    3. Open the throttle valve enough to allow the fast idle cam follower to clear the fast idle cam.
    4.  
    5. The end of the vacuum break rod should be at the outer end of the slot in the vacuum break diaphragm plunger.
    6.  
    7. The specified clearance should register from the lower end of the choke valve to the inside air horn wall.
    8.  
    9. If the clearance is not correct, bend the vacuum break link.
    10.  

    1977 MODELS

    Adjustment procedures for 1978 and later models require the use of an expensive and sophisticated special tool, so procedures are not included here.

    1. Remove the choke thermostatic cover. Place the fast idle cam follower on the high step of the cam.
    2.  
    3. Where there is a purge bleed hole on the back of the choke vacuum break, put tape over the hole. Then, set the diaphragm using an outside vacuum source.
    4.  
    5. Push the inside choke coil lever counterclockwise until the tang on the vacuum break lever touches the tang on the vacuum break plunger stem.
    6.  
    7. Place a gauge of the following diameter between the upper edge of the choke butterfly and the inside wall of the air horn:
    8.  



    California Engines: 0.165"
     
    All Other Engines: 0.160"
     

    1. If the dimension is incorrect, adjust the screw on the vacuum break plunger stem until all play is taken up and choke butterfly just touches the gauge when it's held vertically.
    2.  
    3. Reconnect the vacuum line to vacuum break port of carburetor, remove tape from purge hole, if applied and reinstall and adjust choke thermostat.
    4.  

    Choke Unloader

    Performing this adjustment on 1978 and later models requires sophisticated special tools, so the procedure is not included here.

    On 1977 models, make sure the choke thermostatic spring is properly adjusted (see above). 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 models, use the top side. Bend the fast idle lever tang to achieve the proper opening of the choke.

    Secondary Lockout
    1968-76 MODELS

    Completely open the choke valve and rotate the vacuum break lever clockwise. Bend the lever if the measurement between the lever and the secondary throttle exceeds specifications. Close the choke and gauge the distance between the lever and the secondary throttle shaft pin. Bend the lever to adjust.

    1977-78 MODELS


    Click image to see an enlarged view

    Fig. Fig. 5 Adjusting the secondary lockout on the Rochester 4MC, 4MV and M4MC carburetors

    Please refer to the illustration that follows for this procedure.

    Air Valve Spring

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

    Secondary Closing Adjustment

    This adjustment assures proper closing of the secondary throttle plates.

    1. Set the idle as per instructions in the appropriate section. Make sure that the fast idle cam follower is not resting on the fast idle cam.
    2.  
    3. There should be 0.020" clearance between the secondary throttle actuating rod and the front of the slot on the secondary throttle lever with the closing tang on the throttle lever resting against the actuating lever.
    4.  
    5. Bend the tang on the primary throttle actuating rod to adjust.
    6.  

    Secondary Opening Adjustment
    1. Open the primary throttle valves until the actuating link contacts the upper tang on the secondary lever.
    2.  
    3. With 2-point linkage, the bottom of the link should be in the center of the secondary lever slot.
    4.  
    5. With three point linkage, there should be 0.070" clearance between the link and the middle tang.
    6.  
    7. Bend the upper tang on the secondary lever to adjust as necessary.
    8.  

     
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