Carburetor - Rochester m4mc & m4me 4-bbl GMC Vandura I

COMPONENT TESTING

Note. Specific component testing information not available. Refer to any tests performed during REMOVAL & INSTALLATION or OVERHAUL procedures

APPLICATION

CHEVROLET & GMC (ROCHESTER) CARBURETOR NO.

IDENTIFICATION

The Rochester M4MC and M4ME carburetor numbers are stamped vertically on the float bowl, near the secondary throttle. If float bowl is replaced, follow manufacturer's instructions contained in service package to transfer part number to new float bowl. (Scheme 41)

Carburetor Part Number Location. Scheme 41

DESCRIPTION

The M4MC carburetor is a 2-stage downdraft design. The primary side has a triple venturi system. The secondary side is composed of 2 large bores using the air valve principle (fuel is metered in direct proportion to amount of air passing through secondary bores).

A baffle is attached to secondary side of the air horn above main well bleed tubes. This baffle deflects incoming air to improve secondary nozzle operation during heavy acceleration. The M4MC uses a bowl-mounted choke housing with thermostatic control assembly.

Model M4ME is used for light duty emission vehicles. The M4ME is basically the same carburetor as the M4MC. The M4ME does not have an aneroid cavity and is equipped with an electric rather than hot air choke.

ADJUSTMENT

Note. For all on-vehicle adjustments, see TUNE-UP SERVICE PROCEDURES.

ANGLE GAUGE ADJUSTMENT TOOL

Manufacturer recommends that some carburetor adjustments be performed using a choke valve angle gauge (Kent-Moore tool no. J-26701). While preparations and actual adjustments may vary with each individual adjustment, the procedure for using the angle gauge to check the choke valve angle remains the same. Use the following procedure to perform adjustments requiring the use of the choke angle gauge.

1) Rotate degree scale on angle gauge so that 0° mark is opposite pointer.

2) With choke valve closed, place angle gauge magnet squarely on choke valve. Rotate leveling bubble on angle gauge until it is centered. Rotate degree scale until specified degree mark is opposite pointer. (Scheme 42)

3) Perform individual adjustment preparations as outlined in the following carburetor adjustments requiring angle gauge. If bubble is centered, adjustment is correct. If not, adjust carburetor as outlined.

Choke Valve Angle Gauge. Scheme 42

FLOAT LEVEL

1. Remove air horn. Remove gasket from main body casting. Hold float retainer firmly in place.
2. Using light finger pressure, gently push float against needle. Using a "T" scale, measure distance from top of casting to top of float. Gauging point should be 3/16" back from end of float at toe. (Scheme 43)
3. If adjustment is needed, remove float from main body of carburetor. Bend float arm up or down. Install float and recheck float level.
4. Be sure to check float alignment after adjusting operation. Install new gasket and reinstall air horn.

Float Level Adjustment. Scheme 43

Scheme 44

1. Close throttle valves completely. Make sure fast idle cam follower is off fast idle cam steps. Bend secondary throttle closing tang to fully close primary throttle valves. Readjust after accelerator pump adjustment. (Scheme 44): Accelerator Pump Adjustment
2. Make sure accelerator pump rod is in specified hole (inner or outer) of accelerator pump lever.
3. Using a "T" scale, measure specified distance from top of choke valve wall (next to vent stack) to top of pump stem.
4. To adjust, bend accelerator pump lever at point shown in Fig. (Scheme 44).

CHOKE COIL LEVER

Note. Choke coil cover is retained on housing with rivets to prevent tampering with factory adjustment. If necessary to remove cover, refer to Disassembly and Reassembly procedures in this article. If rivets and cover are removed, a choke thermostat cover retainer kit is required for reassembly.

Scheme 45

1. Remove choke thermostatic cover from choke housing. Place fast idle speed screw on high step of fast idle cam. Push up on thermostatic coil tang (counterclockwise) until choke valve is fully closed.
2. Insert specified drill or pin gauge in hole provided in choke housing. Lower edge of choke lever (inside housing) should just touch drill or pin gauge. (Scheme 45) (Scheme 45): Choke Coil Lever Adjustment
3. To adjust, bend choke rod at point shown in Fig. (Scheme 45). Reinstall choke cover and adjust.

CHOKE ROD (FAST IDLE CAM)

Note. Fast idle adjustment (bench setting) and choke coil lever must be adjusted first. Adjustment is performed using choke valve angle gauge, see procedure at beginning of Adjustment.

1. Place fast idle speed cam follower on 2nd step of fast idle cam against shoulder of highest step.
2. Close choke by pushing up on choke coil lever or vacuum break lever tang. Hold choke closed with a rubber band. (Scheme 46)
3. Bubble on choke angle gauge should be centered with specified angle mark opposite pointer.
4. To adjust, bend tang on fast idle cam until bubble of choke valve angle gauge is centered.

Choke Rod (Fast Idle Cam) Adjustment. Scheme 46

Scheme 47

1. Using an outside vacuum source, seat primary (front) choke vacuum break diaphragm. Plug purge bleed hole (if equipped) with masking tape. Hole is found in end of diaphragm.
2. Make sure air valve is completely closed. Insert a.025" drill or pin gauge between rod and end of slot in lever. (Scheme 47) (Scheme 47): Front Air Valve Rod Adjustment
3. Bend rod at point shown in Fig. (Scheme 47) to adjust clearance in slot. Remove tape and reconnect vacuum hose to diaphragm.

Federal M4ME Only

1. Using an outside vacuum source, seat secondary (rear) choke vacuum break diaphragm.
2. Make sure air valve is completely closed. Insert a.025" drill or pin gauge between rod and end of slot in lever. (Scheme 48)
3. Bend rod at point shown in Fig. (Scheme 48) to adjust clearance in slot. Reconnect vacuum hose to diaphragm.

Rear Air Valve Rod Adjustment. Scheme 48

PRIMARY VACUUM BREAK

Note. Choke coil lever adjustment must be correct before performing this adjustment. This adjustment is performed using the choke angle gauge, see procedure at beginning of Adjustment.

Scheme 49

1. Using an outside vacuum source of at least 15 in. Hg, seat primary vacuum break diaphragm. Plug purge bleed hole (if equipped) with masking tape. Hole is found in end of diaphragm.
2. Air valve rod must not keep vacuum unit from fully retracting. If necessary, bend air valve rod at air valve lever to provide clearance for proper adjustment. Adjust final air rod clearance after setting primary vacuum break adjustment.
3. Lightly close choke by pushing up on choke coil lever or vacuum break lever tang. Hold choke closed with a rubber band. Make sure bucking spring on diaphragm plunger (if equipped) is compressed and seated. Bubble on angle gauge should be centered with specified degree mark opposite pointer. (Scheme 49) (Scheme 49): Primary Vacuum Break Adjustment
4. To adjust, turn vacuum break adjustment screw in until bubble of choke valve angle gauge is centered. Remove gauge.

Note. Some models will have tamper-proof plugs over the adjustment screw. To gain access to adjustment screw, remove vacuum break bracket from carburetor. Carefully grind off plugs over adjustment screw and replace vacuum break diaphragm.

SECONDARY VACUUM BREAK

Note. Choke coil lever adjustment must be correct before performing this adjustment. This adjustment is performed using the choke angle gauge, see procedure at beginning of Adjustment.

Scheme 50

1. Using an outside vacuum source of at least 15 in. Hg, seat secondary vacuum break diaphragm. Plug purge bleed hole (if equipped) with masking tape. Hole is found in end of diaphragm.
2. Air valve rod must not keep vacuum unit from fully retracting. If necessary, bend air valve rod at air valve lever to provide clearance for proper adjustment. Adjust final air rod clearance after setting secondary vacuum break adjustment.
3. Lightly close choke valve by pushing up on choke coil lever or vacuum break lever tang. Hold in position with a rubber band. Make sure bucking spring on diaphragm plunger (if equipped) is compressed and seated. Bubble on choke valve angle gauge should be centered with specified degree mark opposite pointer. (Scheme 50) (Scheme 50): Secondary Vacuum Break Adjustment
4. To adjust, turn screw at end of vacuum diaphragm until bubble in angle gauge is centered. Remove gauge.

AIR VALVE SPRING

1. Using an Allen wrench, loosen lock screw. Turn tension adjusting screw counterclockwise until air valve opens part way. (Scheme 51)
2. Turn tension adjusting screw clockwise until air valve just closes. Then turn adjusting screw clockwise specified number of turns.
3. Hold adjusting screw and tighten lock screw.

Air Valve Spring Adjustment. Scheme 51

AUTOMATIC CHOKE

Note. Choke coil cover is retained on housing by rivets to prevent tampering with factory adjustment. If necessary to remove cover, refer to Disassembly and Reassembly procedures in this article.

CHOKE UNLOADER

Note. This adjustment is performed using the choke valve angle gauge. See procedure at beginning of Adjustments. Choke coil lever must be adjusted correctly, and fast idle adjustment must be set before proceeding.

Choke Unloader Adjustment. Scheme 52

1. If removed, install choke thermostatic cover and coil. Close choke by pushing up on tang on vacuum break lever. Hold in position with a rubber band. Hold primary throttle valves wide open. (Scheme 52)
2. Bubble on choke valve angle gauge should be centered with specified degree mark opposite pointer.
3. To adjust, bend choke unloader tang on throttle lever until bubble of choke valve angle gauge is centered.

Lockout Lever Side Clearance

1. Hold choke valve and throttle valves closed.
2. Measure secondary throttle valve lockout lever side clearance between pin and lockout lever. Bend pin to obtain clearance of.015". (Scheme 53)

Lockout Lever Opening Clearance

1. Push down on tail of fast idle cam to completely open choke valve.
2. Measure secondary throttle valve lockout lever opening clearance between end of pin and toe of lockout lever. (Scheme 53)
3. File end of lockout pin to obtain clearance of.015". Make sure all burrs are removed.

Secondary Throttle Valve Lockout Adjustments. Scheme 53

DISASSEMBLY

Note. Before performing any service on carburetor, it is essential that carburetor is placed on a holding fixture (J-9789-118 or equivalent) to prevent damage to throttle valves.

Idle Speed Solenoid

If equipped with idle speed solenoid, remove screws securing solenoid and bracket to float bowl. Remove assembly.

Air Horn

1. Remove upper choke lever from end of choke shaft by removing retaining screw. Rotate upper choke lever to remove choke rod from slot in lever. Remove choke rod from lower (inner) lever inside float bowl casting. Remove rod by holding lower lever outward with small screwdriver and twisting rod counterclockwise.
2. Remove vacuum hose from primary vacuum break unit. Remove secondary metering rods by removing small screw in top of metering rod hanger. Lift upward on metering rod hanger until secondary metering rods are completely out of air horn. Metering rods may be disassembled from hanger by rotating ends out of holes in end of hanger.
3. Drive pump lever pivot pin inward until pump lever can be removed. Disconnect pump rod from pump lever. Note location of pump rod for reassembly reference.
4. Remove air horn-to-float bowl attaching screws. Countersunk screws (2) are located next to venturi. Remove secondary air baffle deflector (if equipped) from beneath 2 center attaching screws. Remove air horn from float bowl by lifting straight up. Gasket should remain on float bowl for later removal.
5. Remove primary vacuum break attaching screws and vacuum break diaphragm. Disconnect diaphragm from air valve rod and remove rod from air valve lever.
6. Invert air horn to remove pump plunger stem seal (if used). Using a small screwdriver, remove staking holding seal retainer in position. Remove and discard retainer and seal. Use care removing pump plunger stem seal from air horn to prevent damage to air horn casting.
7. It is not necessary to remove choke valve and shaft unless bent or damaged. Choke valve screws are staked in position. Staking must be removed before screws are removed.
8. Further disassembly of air horn is not required. Air valve screws are permanently staked in position. However, a repair kit is available for air valve closing spring and center plastic eccentric cam.

Scheme 54

1. Remove air horn gasket by lifting out of dowel locating pins. Lift tab of gasket from beneath power piston hanger, being careful not to distort springs holding main metering rods.
2. Remove pump plunger and return spring from pump well. Remove power piston and metering rods by depressing piston stem and allowing it to snap free. Repeat until piston force dislodges retainer. DO NOT use pliers on metering rod hanger to remove power piston. Remove power piston spring from well. NOTE: The adjustable part throttle (APT) metering rod adjustment screw is located in a well next to power piston well. The APT is preset at the factory and no attempt should be made to alter its setting. If a new float bowl is required, it will contain a preset APT screw.
3. Remove metering rods from power piston by disconnecting tension spring from top of each rod. Rotate rods out of hanger. (Scheme 54): Exploded View of Rochester M4MC & M4ME Carburetor
4. Remove plastic filler block located over float valve. Remove float assembly and fuel inlet needle by pulling up on retaining pin. Remove inlet seat and gasket. Remove aneroid cavity from float bowl, if equipped.
5. Remove primary main metering jets. Remove pump discharge check ball retainer and check ball. DO NOT remove secondary metering jets. These jets are fixed in place, and if damaged, float bowl must be replaced.
6. Remove secondary air baffle, if replacement is required. Remove pump well baffle. Remove secondary vacuum break attaching screws. Rotate vacuum break assembly to remove vacuum break rod from slot in plunger head. On M4ME models, DO NOT remove the non-adjustable vacuum break link at this time.
7. Align a.159" (No. 21) drill on choke cover retaining rivet and drill only enough to remove rivet head. Repeat for remaining 2 rivets. Drive remainder of rivets out of choke housing. Remove choke cover and coil assembly.
8. Remove choke housing retaining screw located inside choke housing. Slide complete choke assembly from float bowl. Remove plastic tube seal from choke housing (M4MC models). On M4MC models, DO NOT remove baffle plate from below coil on choke cover.
9. On M4ME models, remove non-adjustable vacuum break link. On all models, remove secondary throttle valve lockout lever from float bowl. Remove lower (inside) choke lever from inside float bowl cavity.
10. Remove choke coil lever retaining screw at end of intermediate choke shaft. Remove lever. Slide intermediate choke shaft from choke housing. Remove fast idle cam from intermediate choke shaft. On M4MC models, remove and discard cup seal from from inside choke housing shaft hole.
11. On all models, remove cup seal (intermediate choke shaft) from float bowl insert for bowl cleaning. DO NOT attempt to remove plastic insert.
12. Remove fuel inlet nut, gasket and filter. Remove throttle body-to-float bowl attaching screws and throttle body. Remove throttle body-to-float bowl insulator gasket.

Throttle Body

1. Remove accelerator pump rod from throttle lever by rotating rod until tang on rod aligns with slot in lever.
2. It is not necessary to disassemble throttle body any further. DO NOT remove idle mixture screw plugs unless it is necessary to replace mixture screws or cleaning and air pressure fails to clean idle mixture passages.
3. If necessary to remove idle mixture plugs, proceed as follows: Invert throttle body and position on a holding fixture with manifold side up. Position a punch in between 2 locator points on manifold side of throttle body. There are 2 locator points adjacent to each mixture screw.
4. Using a hammer, drive punch against throttle body to break out portion of throttle body to gain access to idle mixture screw plugs. Drive out hardened steel plugs.
5. Hardened steel plugs will shatter. It is not necessary to remove plug completely. Remove just enough pieces to allow idle mixture adjusting tool (J-28706) or a thin-walled 3/16" deep socket to be used to remove mixture screws and spring.

CLEANING & INSPECTION

1. Use a regular carburetor cleaning solution. Soak components long enough to thoroughly clean all surfaces and passages of foreign matter.
2. Remove any residue after cleaning by rinsing components in a suitable solvent.
3. DO NOT soak any components containing rubber, leather or plastic.
4. Blow out all passages with dry compressed air.

REASSEMBLY

1. Use new gaskets and seals. Make sure that new gaskets fit correctly. Make sure that all holes and slots are punched through and correctly located. To reassemble carburetor, reverse disassembly procedure and note the following
2. Install fuel inlet needle pull clip over edge of flat on float arm facing float. DO NOT hook clip in holes in float arm.
3. Install plastic float bowl filler block after float level adjustment and before metering rod installation.
4. The intermediate choke shaft lever and fast idle cam are installed correctly when tang on lever is beneath fast idle cam. Make choke coil lever adjustments before installing choke coil cover.
5. If choke coil cover was removed, it will be necessary to install replacement rivets supplied in service kit. DO NOT install choke coil cover until completion of adjustments. DO NOT use a gasket between choke housing and choke coil cover on M4ME models. Surface contact is needed to provide a ground for electric choke.
6. Place fast idle screw on high step of fast idle cam. Install choke coil cover, aligning notch in cover with tab on cover retainer (supplied in service kit). Install retainer with tab into screw hole in housing nearest front of carburetor. Install remaining self-tapping screws and tighten.
7. When installing air horn screws, note that 2 long screws are installed with lock washers. Countersunk screws (2) are installed next to venturi area. Install secondary air baffle under screws No. 2 and 4. Tighten air horn screws evenly and in sequence. (Scheme 55)

Air Horn Screw Location and Tightening Sequence. Scheme 55

SPECIFICATIONS

CARBURETOR ADJUSTMENT SPECIFICATIONS