Carburetor - Rochester m4me & m4mef 4-bbl GMC Pickup R1500

COMPONENT TESTING

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

APPLICATION

ROCHESTER CARBURETOR NO.

IDENTIFICATION

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 3)

Scheme 3

DESCRIPTION

The M4M-series carburetors are all 4-barrel, two-stage carburetors. There are 3 major sub-assemblies consisting of an air horn, float bowl and throttle body. 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). All models use an electric choke.

The General Motors M4MEF carburetor has adjustable wide open throttle mixture control. During wide open throttle conditions, a factory-set secondary air bleed adjusting screw provides more precise control of the air/fuel mixture.

ADJUSTMENT

Note. For all on-vehicle adjustments not covered in this article, see appropriate TUNE-UP article.

CHOKE VALVE GAUGE

Some carburetor adjustments are performed using Choke Valve Gauge (J-26701). While preparations and actual adjustments may vary with each individual adjustment, the procedure for using the choke valve gauge remains the same. Use the following procedure to install choke valve gauge

1. Rotate degree scale on choke valve gauge and align zero degree mark with pointer. (Scheme 4) With choke valve completely closed, place choke valve gauge magnet squarely on top of choke valve.
2. Rotate choke valve gauge bubble until it is centered. Rotate degree scale until specified degree mark is opposite pointer. Perform adjustment(s) requiring choke valve gauge.

Scheme 4

VACUUM DIAPHRAGMS

When performing adjustment procedures with vacuum applied to diaphragms, air bleed holes must be plugged. The 3 types of vacuum diaphragms and the proper method for plugging air bleed holes are shown. (Scheme 5)

Scheme 5

FLOAT LEVEL - DRY

1. Remove air horn and gasket. Remove power piston, metering rod assembly and float bowl insert. Attach Float Gauge (J-34817-1) to float bowl. Place Float Weight (J-34817-3) on base with contact pin against outer edge of float lever.
2. Using a "T" scale, measure float level from top of casting to a point 3/16" from large end of float. (Scheme 6) Adjust float level if more than 1/16" from specification.
3. Adjust level by bending float level with Pliers (J-34817-25). Remove tool. Check float level. Repeat procedure until correct level is obtained.

Scheme 6

FLOAT LEVEL - WET

1. With engine idling and choke wide open, insert Float Level Gauge (J-34935-1) in vent hole and allow gauge to float freely. DO NOT press down on gauge as flooding or float damage may result.
2. Observe mark on gauge that lines up with top of casting. Float level must be within 1/16" of specified float level setting. If float level is incorrect, remove air horn and adjust float level. See «FLOAT LEVEL - DRY»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__float-level-dry) .

Note. Incorrect fuel pressure will adversely affect fuel level reading.

Scheme 7

ACCELERATOR PUMP

1. With accelerator pump rod in specified hole of accelerator pump lever, make sure fast idle cam follower is off fast idle cam. Turn throttle stop screw out until it no longer contacts throttle lever.
2. Using a "T" scale, measure distance from top of choke valve wall (next to vent stack) to top of pump stem. To adjust, support accelerator pump lever with screwdriver and bend at notch. (Scheme 7)

Scheme 8

1. Drill out rivets and remove choke cover. Place fast idle speed screw on high step of fast idle cam. Push up on coil lever until choke valve is closed.
2. Insert a.120" drill or pin gauge in hole provided in choke housing. Lower edge of choke lever (inside housing) should just contact drill or gauge. (Scheme 8)
3. To adjust, bend top of choke rod. (Scheme 8) Install choke cover using choke cover kit.

FAST IDLE CAM POSITION

1. Attach a rubber band to tang of intermediate choke shaft. (Scheme 9) Open throttle and allow choke valve to close. Install choke valve gauge and set to specification. See «CHOKE VALVE GAUGE»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__choke-valve-gauge).
2. Place fast idle cam follower on second step of fast idle cam against shoulder of highest step. If cam follower does not contact arm, turn fast idle speed screw until it does.
3. Bubble in choke valve gauge should be centered. If not, bend tang on fast idle cam until bubble is centered.

Choke Rod (Fast Idle Cam) Adjustment. Scheme 9

AIR VALVE ROD - FRONT

1. Plug diaphragm air bleed hole. See «VACUUM DIAPHRAGMS»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__vacuum-diaphragms). Using a hand-held vacuum pump, seat front choke vacuum break diaphragm. Make sure air valve is completely closed.
2. Insert a.025" drill or pin gauge between rod and end of slot in lever. (Scheme 10) Bend rod to adjust clearance. Remove tape and connect vacuum hose to diaphragm.

Scheme 10

AIR VALVE ROD - REAR

1. Plug diaphragm air bleed hole. See «VACUUM DIAPHRAGMS»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__vacuum-diaphragms). Using a hand-held vacuum pump, seat rear choke vacuum break diaphragm. Make sure air valve is completely closed.
2. Insert a.025" drill or pin gauge between rod and end of slot in lever. (Scheme 11) Bend rod to adjust clearance. Remove tape and connect vacuum hose to diaphragm.

Scheme 11

PRIMARY (FRONT) VACUUM BREAK

1. With rubber band attached to tang of intermediate choke shaft, open throttle and allow choke valve to close. Install choke valve gauge and set to specification. See «CHOKE VALVE GAUGE»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__choke-valve-gauge).
2. Plug diaphragm air bleed hole. See «VACUUM DIAPHRAGMS»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__vacuum-diaphragms). Using a hand-held vacuum pump, seat primary vacuum break diaphragm. Bucking spring, if equipped, must be seated against lever. (Scheme 12)
3. To adjust, turn vacuum break adjustment screw until bubble of choke valve gauge is centered. Remove gauge.

Scheme 12

SECONDARY (REAR) VACUUM BREAK

1. Attach rubber band to tang of intermediate choke shaft. Open throttle to allow choke valve to close. Install choke valve gauge and set angle to specification. See «CHOKE VALVE GAUGE»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__choke-valve-gauge).
2. Plug diaphragm air bleed hole. See «VACUUM DIAPHRAGMS»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__vacuum-diaphragms). Using a hand-held vacuum pump, seat secondary vacuum break plunger. If air valve rod prevents plunger from retracting fully, bend rod to permit full plunger travel. (Scheme 13)
3. On some models, use a 1/8" Allen wrench to turn adjustment screw in end of vacuum break. On other models, support vacuum break rod and bend rod at point shown. (Scheme 13) Adjust final rod clearance after rear vacuum break adjustment has been made.

Rear Vacuum Break Adjustment. Scheme 13

AIR VALVE SPRING

1. Using a 3/32" Allen wrench, loosen lock screw. Turn tension adjustment screw counterclockwise until air valve opens. (Scheme 14) Turn tension adjustment screw clockwise until air valve just closes.
2. Turn adjusting screw clockwise the specified number of turns. Hold adjustment screw and tighten lock screw. Apply lithium base grease to spring contact area.

Scheme 14

CHOKE UNLOADER

1. Attach rubber band to tang of intermediate choke shaft. Open throttle to allow choke valve to close. Install choke valve gauge and set angle to specification. See «CHOKE VALVE GAUGE»(/gmc/pickup-r1500/1987-1987/remont/testing-diagnostics/#carburetor-rochester-m4me-m4mef-4-bbl__choke-valve-gauge).
2. Hold secondary lock-out lever away from pin. Hold throttle lever in wide open position. (Scheme 15) To adjust, bend fast idle lever tang until bubble of choke valve gauge is centered.

Choke Unloader Adjustment. Scheme 15

Lock-Out Lever Side Clearance

Close choke valve and throttle valves. Measure side clearance between pin and lock-out lever. Bend pin to obtain clearance of.015" (.38 mm). (Scheme 16) Perform opening clearance adjustment.

Lock-Out Lever Opening Clearance

Push down on fast idle cam to completely open choke valve. Measure opening clearance between end of pin and toe of lock-out lever. (Scheme 16) File end of lock-out pin to obtain clearance of.015" (.38 mm). Make sure all burrs are removed.

Scheme 16

Vacuum Break Assemblies

1. Remove vacuum break hoses and attaching screws. Remove front (primary) vacuum break assembly and air valve lever link.
2. Rotate rear (secondary) vacuum break assembly to remove link from slot. Remove rear vacuum break assembly, air valve lever link and vacuum break-to-choke link.

Air Horn

1. Remove upper choke lever retaining screw and choke lever. Disconnect choke rod from lower lever inside float bowl casting by holding lower lever outward with small screwdriver and twisting rod counterclockwise.
2. Remove Torx head retaining screw from secondary metering rod hanger. Lift out hanger and secondary metering rods as an assembly.
3. Drive pump lever pivot pin inward until pump lever can be removed. Note location of pump rod for reassembly reference. Disconnect pump rod from pump lever.
4. Remove air horn 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.
5. Invert air horn to remove pump plunger stem seal (if equipped). Using a small screwdriver, remove staking holding seal retainer in position. Remove and discard retainer and seal. Use care when removing stem seal to prevent damage to air horn casting.
6. Further disassembly of air horn is not required. If air valve closing spring and/or plastic cam need replacing, a repair kit is available.

Float Bowl

1. Remove and discard float bowl gasket. Remove 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 accelerator 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: DO NOT remove adjustable part throttle (APT) metering rod adjustment screw. Screw is factory set 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. Note position of spring for reassembly reference. Rotate rods out of hanger.
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.
5. Remove aneroid cavity insert. Remove primary (main) metering jets. DO NOT remove secondary jets as they are a permanent part of float bowl. If secondary jets are damaged, complete float bowl must be replaced.
6. Remove pump discharge check ball retainer. Invert bowl and catch check ball.
7. On all models, remove pump well baffle. Using a 5/32" (No. 21) drill bit, drill on rivet heads, just enough to remove heads. Remove choke retainers and choke cover.
8. Use a drift and small hammer to remove remaining pieces of rivets from choke housing. Remove choke mounting screw and washer from inside choke housing and slide choke assembly out of float bowl.
9. Remove secondary lock-out lever from float bowl. Disconnect vacuum link from lever. Remove choke coil lever attaching screw and choke coil lever. Remove intermediate choke shaft, lever and link assembly.
10. Remove fast idle cam assembly. Remove intermediate choke shaft seal from float bowl insert for bowl cleaning. DO NOT attempt to remove plastic insert.
11. Remove fuel inlet nut, gasket, filter and spring. Remove throttle body attaching screws, throttle body and gasket.

Throttle Body

1. Remove accelerator pump rod from throttle lever by rotating rod until tang on rod aligns with slot in lever. Place carburetor in holding fixture, manifold side up, to prevent damage to throttle valves.
2. Using a hacksaw, make 2 parallel cuts in throttle body, one on each side of locator point. (Scheme 17) Cuts should be down to plug, but must not extend more than 1/8" past locator points.
3. Place a small, flat punch at a point near end of saw marks. Hold punch at a 45 degree angle and drive into throttle body until casting breaks away. Plug should be exposed.
4. Hold a center punch vertically and drive into plug. Change punch angle to 45 degrees and drive plug out of casting. Plug will shatter. It is not necessary to remove plug completely, only enough to allow access to idle mixture screw.
5. Count number of turns required to lightly seat idle mixture screws and record for reassembly reference. Remove idle mixture screws. DO NOT disassemble throttle body. Throttle body is serviced as a complete assembly.

Remove Idle Mixture Needle Plugs. Scheme 17

CLEANING & INSPECTION

1. Clean all metal carburetor parts in standard carburetor cleaning solvent. DO NOT soak any rubber or plastic parts, solenoids, pump plunger, pump stem seal, etc., in solvent as they will swell, harden or distort.
2. Plastic cam on air valve shaft and bushing in fuel bowl may be cleaned in carburetor cleaner. Rinse thoroughly after cleaning.
3. Thoroughly clean all metal parts and blow dry with shop air. Make sure all passages and metering parts are free of burrs and dirt.
4. DO NOT use drills or wire to clean jets.
5. Power valve piston bore in float bowl may be cleaned with a .375" soft-wire brush. Turn brush clockwise through full length of bore, plus several more turns to remove dirt and varnish.
6. Inspect carburetor surfaces for damage. Inspect holes in levers for excessive wear or out-of-round condition. Replace levers, if worn. Inspect plastic parts for cracks, damage, etc.

Note. The letters "MW" on float bowl, if present, indicate a Machined Pump Well. If replacement of float bowl is required, new float bowl must also display these letters.

REASSEMBLY

1. To assemble carburetor, reverse disassembly procedure. 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.
2. Turn idle mixture adjusting screws in until lightly seated, then back out number of turns recorded during disassembly. If number is unknown, back out 3 turns as a preliminary adjustment.
3. Install fuel inlet needle pull clip over edge of flat on float arm facing float. DO NOT hook clip in holes in float arm. Install plastic float bowl filler block after float level adjustment and before metering rod installation.
4. When installing new pump plunger stem seal and retainer in air horn, lip on seal faces out. Lightly stake seal retainer in place at 3 locations, different from original stakings.
5. When installing air horn screws, countersunk screws (2) are installed next to venturi area. Install secondary air baffle under screw Nos. 2 and 4. Tighten air horn screws evenly and in sequence. (Scheme 18)
6. Install accelerator pump rod link in hole of pump lever as noted during disassembly. Intermediate choke shaft lever and fast idle cam are installed correctly when tang ion lever is beneath fast idle cam.
7. Make all choke coil lever adjustments before installing choke cover assembly. Place fast idle screw on high step of fast idle cam and be sure that coil tang engages inside coil pick-up lever. Install choke cover with rivets supplied in kit.

Scheme 18

Scheme 19

SPECIFICATIONS

CARBURETOR ADJUSTMENT SPECIFICATIONS

See also:
• FLOAT LEVEL - DRY
• CHOKE VALVE GAUGE
• VACUUM DIAPHRAGMS