Carburetor - Rochester 1mef 1-bbl GMC Cab & Chassis R2500

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 model identification is stamped vertically on float bowl, adjacent to the fuel inlet nut. If replacing float bowl, follow manufacturer's instructions contained in service package so that the identification number can be transferred to the new float bowl. (Scheme 73)

Scheme 73

DESCRIPTION

Model 1MEF is single stage, single barrel carburetor. The "E" in model designation indicates an electrically-activated integral automatic choke system and the "F" indicates an adjustable wide open throttle mixture control.

The choke vacuum diaphragm is mounted externally to carburetor air horn. It is connected to the thermostatic coil lever through a connector link. Carburetor uses an electrically actuated idle stop solenoid. A pre-set metering rod adjusting screw is located in the air horn.

ADJUSTMENT

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

FLOAT LEVEL

1. Remove air horn and gasket. Hold float pin firmly in place. Push end of float arm down against top of float needle. (Scheme 74)
2. Using depth gauge or "T" scale, measure distance from top of casting to index point at toe of float.
3. To adjust, gently bend float arm up or down. To avoid damage, do not force needle against needle seat.

Scheme 74

METERING ROD

1. To remove metering rod, hold throttle valve wide open. Push metering rod down against spring tension. Slide metering rod out of slot in holder and remove from main metering jet.
2. Hold throttle valve fully closed and back out idle stop solenoid. Hold power piston down. Swing metering rod holder over flat surface of bowl casting next to carburetor bore with gasket removed.
3. Measure specified clearance between rod holder and carburetor surface. Measurement can be made using a specified drill or pin gauge. Clearance should be a "slide" fit. (Scheme 75)
4. To adjust, gently bend holder arm up or down. Recheck clearance. Reassemble carburetor and install new air horn gasket.

Scheme 75

CHOKE COIL LEVER

1. Place fast idle cam follower on highest step of fast idle cam. Hold choke valve closed. (Scheme 76)
2. If adjustment is correct, specified plug gauge should pass through hole in lever and enter hole in casting. To adjust, bend connector link.

Scheme 76

FAST IDLE CAM POSITION (CHOKE ROD)

1. Make sure fast idle speed is correctly set. Hold fast idle cam follower on second step of fast idle cam, against highest step. (Scheme 77)
2. Apply light closing pressure to choke valve. Measure specified clearance between lower edge of choke valve (center) and air horn wall. Measurement can be made with a specified drill or pin gauge. To adjust, bend fast idle cam rod.

Scheme 77

AUTOMATIC CHOKE

Note. Choke coil cover uses rivets in place of retaining screws. If necessary to remove choke coil cover, refer to AIR HORN in DISASSEMBLY under OVERHAUL in this article.

VACUUM BREAK

1. Place fast idle cam follower on highest step of fast idle cam. Using an outside vacuum source, apply enough vacuum to seat diaphragm. Push down on choke valve. Diaphragm plunger should be seated and the bucking spring should be compressed (if equipped). (Scheme 78)
2. Measure clearance between lower edge of choke valve and inside air horn wall, using specified drill or pin gauge. On models equipped with delay feature, cover plug and purge bleed hole in vacuum break end cover with masking tape.
3. On all models, to adjust, bend "U" shaped portion of vacuum diaphragm connector link. Remove masking tape. Check linkage for binding and freedom of movement.

Scheme 78

CHOKE UNLOADER

1. Install choke coil in housing and index properly. If choke is warm, cool down to point where choke valve will fully close.
2. Hold throttle valve wide open. Measure clearance between lower edge of choke valve and inside air horn wall, using specified drill or pin gauge. To adjust, bend choke unloader tang to achieve specified clearance. (Scheme 79)

Scheme 79

Idle Mixture Needle Plug Removal

1. Turn carburetor over and drain fuel. Place carburetor in holding fixture, manifold side up.
2. Using a hacksaw, make 2 parallel cuts in the throttle body, one on each side of locator point. (Scheme 80) Cuts should be down to plug, but must not extend more than 1/8" past locator point.
3. Place a small flat punch at a point near ends 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 of it to allow access to idle mixture screw.

Scheme 80

Air Horn

1. Place carburetor on suitable stand to prevent damage to throttle valve. Remove idle stop solenoid and solenoid spring. Pull off vacuum break diaphragm hose. Remove 2 diaphragm assembly attaching screws. Remove diaphragm assembly and choke connector bracket.
2. Slide diaphragm plunger stem from choke lever link. DO NOT attempt to remove screw that retains vacuum break lever to choke shaft. This screw is installed with thread-locking compound. It should not be removed unless choke shaft replacement is required.
3. Remove fast idle cam attaching screw and cam. Remove choke rod from choke coil lever on end of choke shaft. Remove 3 choke coil housing attaching screws from float bowl assembly.
4. If necessary to remove choke coil cover, drill rivet heads from cover retainer using a .159" (4 mm) drill bit. Using a drift and small hammer, drive remainder of rivets out of choke housing. Remove 3 retainers and cover from housing.
5. Remove remaining air horn-to-float bowl screws and lock washers. Carefully remove air horn by lifting and twisting back toward choke housing. Disengage choke coil lever link from choke coil lever at choke housing.
6. Choke valve retaining screws are staked into place. Choke valve should not be removed. DO NOT turn or remove metering rod adjusting screw. This could cause engine damage and/or increased emissions.

Float Bowl

1. Remove air horn gasket. Lift up on float hinge pin to remove float assembly from bowl. Remove hinge pin from float arm. Withdraw float needle from seat.
2. Remove accelerator pump lever retaining screw. Hold pump rod and power piston assembly down. Disconnect link from power piston rod. Disconnect pump link from pump rod.
3. Remove accelerator pump rod, pump assembly and return spring. Remove pump rod seal.
4. Remove power piston assembly, power piston rod, metering rod and spring assembly, and power piston spring. Separate metering rod and spring assembly from power piston metering rod hanger.
5. Use needle nose pliers to remove "T" guide and pump discharge spring. Turn bowl over and remove pump discharge ball and idle tube. Remove main metering jets from bottom of fuel bowl.
6. Remove float needle seat and gasket. Remove fuel inlet nut, filter and spring. No further disassembly is required.

Scheme 81

Throttle Body

1. Invert float bowl on bench. Remove throttle body-to-bowl attaching screw. Remove insulator and gasket. Further disassembly of throttle body is not required, unless idle mixture needle is damaged or idle channels need cleaning.
2. If idle mixture needle must be removed, turn mixture screw in until needle is lightly seated, counting number of turns. Record for reassembly reference. Remove needle.

Note. Due to close tolerance fit of throttle valve in bore of throttle body, throttle valve or shaft must NOT be removed.

CLEANING & INSPECTION

1. Clean all metal carburetor parts in regular (cold) carburetor cleaning solution. DO NOT soak any rubber or plastic parts in cleaner.
2. Use compressed air to blow out all passages in castings. DO NOT use drills to clean any passages.
3. Inspect needle and seat, all levers and links, fast idle cam, return springs, etc. for damage and/or excessive wear. Replace as needed. If float needle and seat are worn, install a new factory matched set.
4. Inspect upper and lower casting gasket surfaces for damage.

REASSEMBLY

Use new gaskets, seals and filter. Make sure that new gaskets fit correctly, and that all holes and slots are punched through and correctly located. To reassemble carburetor, reverse disassembly procedure.

1. When installing needle and seat assembly, turn in until lightly seated, then back out the number of turns recorded during disassembly. If unknown, back out 2 turns (final adjustment will be made on vehicle).
2. Install new power piston rod seal. Use small screwdriver to install retainer, and tap into place until flush with casting surface.
3. When assembling power rod link and accelerator pump link on pump and power rod lever: Lever offset should face throttle body. Bend in accelerator pump link should face fuel inlet and ends of link should face throttle body. Bend in power piston rod link should face away from fuel inlet and end of link with squirt (bump) should face away from throttle body.
4. When float bowl assembly is complete, perform float level and metering rod adjustments.
5. When attaching air horn to float bowl, tighten screws in order. (Scheme 82) Numbers on fast idle cam face outward. When installing choke cover and coil assembly in choke housing, align notch in cover with raised casting projection on housing cover flange.
6. Make sure coil pick-up tang engages choke coil lever. Use replacement rivets supplied with service kit. DO NOT use gasket between electric coil and housing. Adjust choke coil lever, fast idle cam, vacuum break, and choke unloader.

Scheme 82

SPECIFICATIONS

CARBURETOR ADJUSTMENT SPECIFICATIONS

See also:
• DISASSEMBLY