GM Cadillac 1967-1989 Repair Guide

Rochester 4MV-4 BBL Carburetor

Print

The Rochester Quadrajet carburetor is a two stage, four-barrel downdraft carburetor. The designation 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 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 in. 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.

ACCELERATOR PUMP ADJUSTMENT



  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 ADJUSTMENT



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 plump lever.

FLOAT LEVEL ADJUSTMENT



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.

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 ADJUSTMENT

  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 ADJUSTMENT



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

AIR VALVE DASHPOT ADJUSTMENT



See Figure 1

Set the vacuum break diaphragm. Hold the air valve tightly closed. 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.



Click image to see an enlarged view

Fig. Fig. 1: Air valve dashpot adjustment - 4MV carburetor

CHOKE COIL ROD ADJUSTMENT



  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.  

VACUUM BREAK ADJUSTMENT



  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.  

CHOKE UNLOADER ADJUSTMENT



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. Use the bottom side of the butterfly for this measurement. Bend the fast idle lever tang to achieve the proper opening of the choke.

SECONDARY LOCKOUT ADJUSTMENT



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.

AIR VALVE SPRING ADJUSTMENT



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 in. 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 two 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 in. clearance between the link and the middle tang.
  6.  
  7. Bend the upper tang on the secondary lever to adjust as necessary.
  8.  

 
label.common.footer.alt.autozoneLogo