GM Full Size Vans 1987-1997 Repair Guide

Carburetor

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DESCRIPTION & OPERATION



These Rochester models are four barrel, two stage carburetors with three major assemblies: the air horn, float bowl, and throttle body. They have six basic operating systems:

  1. Float
  2.  
  3. Idle
  4.  
  5. Main Metering
  6.  
  7. Power
  8.  
  9. Pump
  10.  
  11. Choke
  12.  

The first "M" in the model identification number indicates that the carburetor is of a Modified primary metering "open loop" design. The "4M" is the model designation, indicating it is a four barrel carburetor. The remaining letters designate specific features as follows:



C - It has an integral Hot Air Choke.
 
D - Dual capacity pump valve and a combined mixture control/dual capacity pump solenoid assembly.
 
E - Has an electric choke.
 
F - Has an adjustable wide open throttle mixture control.
 

The carburetor identification number is stamped vertically on the float bowl near the secondary throttle lever. Refer to this number before servicing the carburetor. If replacing the float bowl assembly, follow the instructions in the service package and stamp or engrave the number on the new float bowl.

A single float chamber supplies fuel to all carburetor bores. A float, float needle with pull clip and a float needle seat are used to control the level of fuel in the float chamber. A vacuum-operated power piston and metering rods control the air/fuel metering in the primary bores of the carburetor. Tapered metering rods are attached to the power valve piston assembly and move in fixed metering jets to provide the fuel flow for varying engine demands. A factory-set adjustable part throttle screw, used on all models, precisely positions the tapered portion of the metering rods in the jets. On M4MEF models, the factory-set rich stop adjusting bushing precisely positions the enrichment portion of the metering rods in the jets.

Air valves and tapered metering rods control the air/fuel mixture in the secondary bores during increased engine air flow at wide open throttle. On M4MEF models, the factory-set secondary well air bleed adjusting screw provides additional control of the air/fuel mixture during Wide Open Throttle (WOT).

The accelerator pump system on all models uses a throttle actuated pump plunger, operating in the pump well. The pump provides extra fuel during quick throttle openings.

An electrically heated choke coil provides the choke valve closing force for cold startup and for correct opening time during warm-up. A vacuum break assembly(s) controls initial choke valve opening at startup to provide sufficient air flow to the engine. An unloader tang on the throttle lever forces the choke valve to open to purge a flooded engine when the accelerator is pressed to the floor. The fast idle cam, following choke valve movement, acts as a graduated throttle stop to provide increased idle speed during warm-up.

The electric Idle Stop Solenoid (ISS) on Federal V8 engines with manual transmission provides the desired engine idle speed, and prevents dieseling when the ignition is switched off. A vacuum-operated Throttle Kicker assembly on California V8 engines retards throttle closing during deceleration to improve emission control. Vacuum to the kicker is controlled by the Throttle Return Control system.

TESTING



Float Level (External)

This procedure requires the use of an external float level gauge tool No. J-34935-1 or equivalent.

  1. With the engine idling and the choke valve wide open, insert tool J-34935-1 in the vent slot or vent hole. Allow the gauge to float freely.
  2.  

Do not press down on the gauge. Flooding or float damage could result.

  1. Observe the mark on the gauge that lines up with the top of the casting. The setting should be within 1 / 16 in. (1.6mm) of the specified float level setting. Incorrect fuel pressure will adversely affect the fuel level.
  2.  
  3. If not within specification, remove the air horn and adjust the float.
  4.  

Choke (Engine Off)
  1. Remove the air cleaner assembly.
  2.  
  3. Hold the throttle half way down.
  4.  
  5. Open and close the choke several times. Be sure all links are connected and not damaged. Choke valve, linkage, and fast idle cam must operate freely.
  6.  
  7. If the choke valve, linkage or fast idle cam is sticking due to varnish, clean with choke cleaner.
  8.  
  9. Do not lubricate linkage, as lubricant will collect dust and cause sticking.
  10.  

Electric Choke (On Vehicle)
  1. Allow the choke thermostat to stabilize at about 70°F (21°C).
  2.  
  3. Open the throttle to allow the choke valve to close.
  4.  
  5. Start the engine and determine the length of time for the choke valve to reach full open position:
    1. If longer than five minutes, check the voltage at the choke stat connector, with the engine running.
    2.  
    3. If voltage is between 12 and 15 volts, check for proper ground between choke cover and choke housing. If correct, replace choke cover assembly.
    4.  
    5. If the voltage is low or zero, check all wires and connections.
    6.  

  6.  

Vacuum Break

A hand-operated vacuum pump such as J-23738-A or equivalent will be needed for this procedure.

  1. If the vacuum break has an air bleed hole, plug it during this checking procedure.
  2.  
  3. Apply 15 in. Hg (50 kPa) vacuum to the vacuum break with the hand pump.
    1. Apply finger pressure to see if the plunger has moved through full travel. If not, replace the vacuum break.
    2.  
    3. Observe the vacuum gauge. Vacuum should hold vacuum for at least twenty seconds. If not, replace the vacuum break.
    4.  

  4.  
  5. Replace vacuum break hoses that are cracked, cut or hardened.
  6.  

Idle Stop Solenoid (ISS)

A non-functioning idle stop solenoid (if equipped) could cause stalling or rough idle.

  1. Turn the ignition ON , but do not start the engine.
  2.  
  3. Open the throttle momentarily to allow the solenoid plunger to extend.
  4.  
  5. Disconnect the wire at the solenoid. The plunger should drop back from the throttle lever.
  6.  
  7. Connect the solenoid wire. The plunger should move out and contact the throttle lever.
  8.  
  9. If the plunger does not move in and out as the wire is disconnected and connected, check the voltage across the feed wire:
    1. If 12-15 volts are present in the feed wire, replace the solenoid.
    2.  
    3. If the voltage is low, locate the cause of the open circuit in the solenoid feed wire and repair as necessary.
    4.  

  10.  

Throttle Kicker

A hand-operated vacuum pump such as J-23738-A or equivalent will be needed for this procedure.

  1. Hold the throttle half way open to allow the plunger to extend fully.
  2.  
  3. Apply 20 in. Hg (67 kPa) vacuum to the throttle kicker with the hand vacuum pump.
    1. Apply finger pressure to the plunger to see if it has extended fully. If not, replace the throttle kicker.
    2.  
    3. Observe the vacuum gauge. Vacuum should hold for at least twenty seconds. If not, replace the throttle kicker.
    4.  

  4.  
  5. Release the vacuum to the throttle kicker.
  6.  
  7. If the plunger does not retract to its starting position, replace kicker.
  8.  

ADJUSTMENTS



Float

See Figures 1, 2 and 3



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Fig. Fig. 1: Carburetor adjustment specifications - M4ME models



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Fig. Fig. 2: Carburetor adjustment specifications - M4ME models (continued)



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Fig. Fig. 3: Use a T-scale to adjust the float level

A float level T-scale such as J-9789-90 or equivalent and float positioning tool kit J-34817 or equivalent will be needed for this adjustment.

  1. Remove the air horn, gasket, power piston and metering rod assembly, and the float bowl insert.
  2.  
  3. Attach float adjustment tool J-34817-1 or equivalent to the float bowl.
  4.  
  5. Place float adjustment tool J-34817-3 or equivalent into float adjustment tool J-34817-1 or equivalent, with the contact pin resting on the outer edge of the float lever.
  6.  
  7. Measure the distance from the top of the casting to the top of the float at a point 3 / 16 in. (2mm) from the large end of the float using the float adjustment tool J-9789-90 or equivalent.
  8.  
  9. If more than 1 / 16 in. (1.6mm) from specification, use the float gauge adjusting tool J-34817-25 or equivalent to bend the lever up or down. Remove the gauge adjusting tool and measure, repeating until within specification.
  10.  

Pump

See Figure 4



Click image to see an enlarged view

Fig. Fig. 4: Typical pump adjustment

Float Level T-scale J-9789-90 or equivalent will be needed for this adjustment.

  1. The pump link must be in the specified hole to make this adjustment.
  2.  
  3. With the fast idle cam off the cam follower lever, turn the throttle stop screw out so it does not touch the throttle lever.
  4.  
  5. Measure the distance from the top of the choke valve wall to the top of the pump stem.
  6.  
  7. Adjust, if necessary, by supporting the pump lever with a screwdriver and bending it at the notch.
  8.  

Air Valve Return Spring

See Figure 5



Click image to see an enlarged view

Fig. Fig. 5: Air valve spring adjustment

  1. Loosen the setscrew.
  2.  
  3. Turn spring fulcrum pin counterclockwise until the air valves open.
  4.  
  5. Turn the pin clockwise until the air valves close, then the additional turns specified.
  6.  
  7. Tighten the setscrew. Apply lithium grease to the spring contact area.
  8.  

Choke Stat Lever

Linkage Bending Tool J-9789-111 or equivalent will be needed for this adjustment.

  1. Drill out and remove the choke cover attaching rivets. Remove the choke cover and thermostat assembly.
  2.  
  3. Place fast idle cam on high step against the cam follower lever.
  4.  
  5. Push up on the choke stat lever to close the choke valve.
  6.  
  7. Check the stat lever for correct orientation by inserting a 0.120 in. (3mm plug gauge hole in the choke housing. The gauge should fit in the hole and touch the edge of the lever.
  8.  
  9. Adjust, if necessary, by bending the choke link with J-9789-111 or equivalent.
  10.  

Choke Link and Fast Idle Cam

See Figures 6 and 7



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Fig. Fig. 6: Fast idle cam adjustment



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Fig. Fig. 7: Choke rod adjustment procedures

Choke Valve Angle Gauge J-26701-A or equivalent will be needed for this adjustment.

  1. Attach a rubber band to the vacuum break lever of the intermediate choke shaft.
  2.  
  3. Open the throttle to allow the choke valve to close.
  4.  
  5. Set up J-26701-A or equivalent and set the angle to specification as follows:
    1. Rotate the degree scale until zero is opposite the pointer.
    2.  
    3. Center the leveling bubble.
    4.  
    5. Rotate the scale to the specified angle.
    6.  

  6.  
  7. Place the fast idle cam on the second step against the cam follower lever, with the lever contacting the rise of the high step. If the lever does not contact the cam, turn the fast idle adjusting screw in additional turn(s).
  8.  

Final fast idle speed adjustment must be performed according to the underhood emission control information label.

  1. Adjust, if bubble is not recentered, by bending the fast idle cam kick lever with pliers.
  2.  

Primary Side Vacuum Break

See Figure 8



Click image to see an enlarged view

Fig. Fig. 8: Primary side vacuum break adjustment

Choke Valve Angle Gauge J-26701-A and Hand Operated Vacuum Pump J-23738-A or equivalents, will be needed for this adjustment.

  1. Attach rubber band to the vacuum break lever of the intermediate choke shaft.
  2.  
  3. Open the throttle to allow the choke valve to close.
  4.  
  5. Set up J-26701-A or equivalent and set angle to specifications.
  6.  
  7. Plug the vacuum break bleed holes, if applicable. Apply 15 in. Hg (50 kpa) vacuum to seat the vacuum break plunger.
  8.  
  9. Seat the bucking spring, if so equipped. If necessary, bend the air valve link to permit full plunger travel, then re-apply vacuum to fully retract the plunger.
  10.  
  11. Adjust, if the bubble is not re-centered, by turning the vacuum break adjusting screw.
  12.  

Secondary Side Vacuum Break

Choke Valve Angle Gauge J-26701-A, Hand Operated Vacuum Pump J-23738-A and Linkage Bending Tool J-9789-111 or equivalents will be needed for this adjustment.

  1. Attach a rubber band to the vacuum break lever of the intermediate choke shaft.
  2.  
  3. Open the throttle to allow the choke valve to close.
  4.  
  5. Set up J-26701-A or equivalent and set angle to specification.
  6.  
  7. Plug vacuum break bleed holes, if so equipped. Apply 15 in. Hg (50 kPa) vacuum to seat the vacuum break plunger.
  8.  
  9. Compress the plunger bucking spring, if so equipped. If necessary, bend the air valve link to permit full plunger travel, then re-apply vacuum to fully retract the plunger.
  10.  
  11. Adjust, if bubble is not re-centered, by either supporting the link where shown and bending it with J-9789-111 or equivalent, or by turning the screw with a 1 / 8 in. hex wrench.
  12.  

Air Valve Link

See Figure 9 and 10



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Fig. Fig. 9: Front air valve rod adjustment



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Fig. Fig. 10: Rear air valve rod adjustment

Hand Operated Vacuum Pump J-23738-A and Linkage Bending Tool J-9789-111 or equivalents will be needed for this adjustment.

  1. Plug vacuum break bleed holes, if applicable. With the air valves closed, apply 15 in. Hg (50 kpa) vacuum to seat the vacuum break plunger.
  2.  
  3. Gauge the clearance between the air valve link and the end of the slot in the air valve lever. Clearance should be 0.025 in. (0.6mm).
  4.  
  5. Adjust, if necessary, by bending the link with J-9789-111 or equivalent.
  6.  

Unloader Adjustment

See Figure 11



Click image to see an enlarged view

Fig. Fig. 11: Choke unloader adjustment

Choke Valve Angle Gauge J-26701-A and Linkage Bending Tool J-9789-111 or their equivalents will be needed for this adjustment.

  1. Attach a rubber band to the vacuum break lever of the intermediate choke shaft.
  2.  
  3. Open the throttle to allow the choke valve to close.
  4.  
  5. Set up J-26701-A or equivalent and set angle to specifications.
  6.  
  7. Hold the secondary lockout lever away from the pin.
  8.  
  9. Hold the throttle lever in wide open position.
  10.  
  11. Adjust, if bubble is not re-centered, by bending fast idle lever with J-9789-111 or their equivalent.
  12.  

Secondary Throttle Lockout
  1. Place the fast idle cam on the high step against the cam follower lever.
  2.  
  3. Hold the throttle lever closed.
  4.  
  5. Gauge the clearance between the lockout lever and pin. It must be 0.010-0.020 in.
  6.  
  7. Adjust, if necessary, by bending the pin.
  8.  
  9. Push down on tail of fast idle cam to move lockout lever away from pin.
  10.  
  11. Rotate the throttle lever to bring the lockout pin to the position of minimum clearance with the lockout lever.
  12.  
  13. Gauge the clearance between the lockout lever and pin. The minimum must be 0.015 in.
  14.  
  15. Adjust, if necessary, by filing the end of the pin.
  16.  

Idle Speed and Mixture

In case of a major carburetor overhaul, throttle body replacement, or high idle CO (when indicated by an emissions inspection), the idle mixture may be adjusted. Adjusting the mixture by other than the following method may violate Federal and/or state laws. Idle mixture needle socket J-29030-B or equivalent is required for this adjustment.

  1. Set the parking brake and block the drive wheels.
  2.  
  3. Remove the carburetor from the engine.
  4.  
  5. Drain the fuel from the carburetor into a container. Dispose of the fuel in an approved container.
  6.  
  7. Remove the idle mixture needle plugs as follows:
    1. Invert the carburetor and support it to avoid damaging external components.
    2.  
    3. Make two parallel hacksaw cuts in the throttle body, between the locator points near one idle mixture needle plug. The distance between the cuts depends on the size of the punch to be used.
    4.  
    5. Cut down to the plug, but not more than 1 / 8 in. beyond the locator point.
    6.  
    7. Place a flat punch at a point near the ends of the saw marks. Hold the punch at a 45° angle and drive it into the throttle body until the casting breaks away, exposing the steel plug.
    8.  
    9. Use a center punch to break the plug apart, uncover idle mixture needle. Remove all loose pieces of plug.
    10.  
    11. Repeat the previous steps for the other needle plug.
    12.  

  8.  
  9. Use idle mixture needle socket J-29030-B or equivalent to lightly seat the idle mixture needles, then back them out three turns.
  10.  
  11. Reinstall the carburetor on the engine.
  12.  
  13. Place the transmission in Park (automatic transmission) or Neutral (manual transmission).
  14.  
  15. Start the engine and bring it to a normal operating temperature, choke valve open, and air conditioning off.
  16.  
  17. Connect a known accurate tachometer to the engine.
  18.  
  19. Check ignition timing, and adjust if necessary, by following the procedure described on the Emission Control Information Label located under the hood on the vehicle.
  20.  
  21. Use idle mixture needle socket J-29030-B or equivalent to turn the mixture needles equally ( 1 / 8 turn at a time), in or out, to obtain the highest rpm (best idle).
  22.  
  23. Adjust the idle speed screw (throttle stop) to obtain the base idle speed specified on the underhood emission control information label.
  24.  
  25. Again try to readjust mixture needles to obtain the highest idle rpm. The adjustment is correct when the highest rpm (best idle) is reached with the minimum number of mixture needle turns from the seated position.
  26.  
  27. If necessary, readjust the idle speed screw (throttle stop) to obtain the specified base idle speed.
  28.  
  29. Check (and if necessary adjust) the idle speed solenoid activated speed and fast idle speed. Refer to the underhood emission control information label.
  30.  
  31. Check the throttle kicker and adjust if necessary.
  32.  
  33. Turn off the engine, remove all test equipment and remove the block from the drive wheels.
  34.  

REMOVAL & INSTALLATION



  1. Disconnect the negative battery terminal. Remove the air cleaner assembly and gasket.
  2.  
  3. Disconnect the electrical connectors from the choke and idle stop solenoid.
  4.  
  5. Disconnect and tag the vacuum hoses.
  6.  
  7. Disconnect the accelerator linkage, downshift cable (automatic transmission only) and cruise control linkage (if equipped).
  8.  
  9. Disconnect the fuel line connection at the fuel inlet nut.
  10.  
  11. Remove the carburetor attaching bolts and the carburetor with the flange insulator.
  12.  


CAUTION
Clean the sealing surfaces on the intake manifold and carburetor. Be sure to extinguish all open flames while filling and testing carburetor with gasoline to avoid the risk of fire.

To install:
  1. Install the carburetor with a new flange gasket. It is good shop practice to fill the carburetor float bowl before installing the carburetor. This reduces the strain on starting motor and battery and reduces the possibility of backfiring while attempting to start the engine. Operate the throttle several times and check the discharge from pump jets before installing the carburetor.
  2.  
  3. Install the carburetor attaching bolts and torque them to 12 ft. lbs. (16 Nm). Be sure to tighten the bolts in a crisscross pattern.
  4.  
  5. Install the fuel line to the fuel inlet nut, cruise control cable (if equipped), downshift cable (automatic transmission only), accelerator linkage, vacuum hoses, electrical connectors to the choke and idle stop solenoid, air cleaner assembly with gasket and the negative battery terminal.
  6.  

After servicing the carburetor, tighten the mounting bolts in a clockwise direction to 12 ft. lbs. (16 Nm). When tightening the carburetor at recommended maintenance intervals, check the bolt torque. If less than 5 ft. lbs. (7 Nm), retighten to 8 ft. lbs. (11 Nm); but if greater than 5 ft. lbs. (7 Nm), do not retighten.

OVERHAUL



See Figures 12 and 13

Efficient carburetion depends greatly on careful cleaning and inspection during overhaul, since dirt, gum, water or varnish in or on the carburetor parts are often responsible for poor performance.

Overhaul your carburetor in a clean, dust-free area. Carefully disassemble the carburetor, referring often to the exploded views and directions packaged with the rebuilding kit. Keep all similar and look-alike parts segregated during disassembly and cleaning to avoid accidental interchange during assembly. Make a note of all jet sizes.

When the carburetor is disassembled, wash all parts (except diaphragms, electric choke units, solenoids, pump plunger and any other plastic, leather, fiber or rubber parts) in clean carburetor solvent. DO NOT leave the parts in the solvent any longer than is necessary to sufficiently loosen the deposits. Excessive cleaning may remove the special finish from the float bowl and choke valve bodies, leaving these parts unfit for service. Rinse all parts in clean solvent and blow them dry with compressed air or allow them to air dry. Wipe clean all cork, plastic, leather and fiber parts with a clean, lint-free cloth.

Blow out all passages and jets with compressed air and be sure that there are no restrictions or blockages. Never use wire or similar tools to clean the jets, fuel passages or air bleeds. Clean all jets and valves separately to avoid accidental interchange.

Check all parts for wear or damage. If wear or damage is found, replace the defective parts. Especially check the following:

  1. Check the float needle and seat for wear. If wear is found, replace the complete assembly.
  2.  
  3. Check the float hinge pin for wear and the float(s) for dents or distortion. Replace the float if fuel has leaked into it.
  4.  
  5. Check the throttle and choke shaft bores for wear or an out-of-round condition. Damage or wear to the throttle arm, shaft or shaft bore will often require replacement of the throttle body. These parts require a close fitting tolerance; wear may allow air leakage, which could affect starting and idling.
  6.  

Throttle shafts and bushings are not included in overhaul kits. They can be purchased separately.

  1. Inspect the idle mixture adjusting needles for burrs or grooves. Any such condition requires replacement of the needle, since you will not be able to obtain a satisfactory idle.
  2.  
  3. Test the accelerator pump check valves. They should pass air one way but not the other. Test for proper seating by blowing and sucking on the valve. Replace the valve check ball and spring as necessary. If the valve is satisfactory, wash the valve parts again to remove breath moisture.
  4.  
  5. Check the bowl cover for warped surfaces with a straightedge.
  6.  
  7. Closely inspect the accelerator pump plunger for wear and damage, replacing as necessary.
  8.  
  9. After the carburetor is assembled, check the choke valve for freedom of operation.
  10.  

Carburetor overhaul kits are recommended for each overhaul. These kits contain all gaskets and new parts to replace those which deteriorate most rapidly. Failure to replace all parts supplied with the kit (especially gaskets) can result in poor performance later.

Some carburetor manufacturers supply overhaul kits of three basic types: minor repair, major repair and gasket kits. Basically, they contain the following:

Minor Repair Kits:


All gaskets
 
Float needle valve
 
All diagrams
 
Spring for the pump diaphragm
 

Major Repair Kits:


All jets and gaskets
 
All diaphragms
 
Float needle valve
 
Pump ball valve
 
Float
 
Complete intermediate rod
 
Intermediate pump lever
 
Some cover hold-down screws and washers
 

Gasket Kits:


All gaskets
 

After cleaning and checking all components, reassembly the carburetor, using new parts and referring to the exploded view. When reassembling, make sure that all screws and jets are tight in their seats but DO NOT overtighten, for the tips will be distorted. Tighten all screws gradually, in rotation. DO NOT tighten the needle valves into their seats; uneven jetting will result. Always use new gaskets. Be sure to adjust the float level when reassembling.



Click image to see an enlarged view

Fig. Fig. 12: Exploded view of the carburetor assembly - M4ME carburetor



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Fig. Fig. 13: Exploded view of the carburetor assembly - M4ME carburetor, continued

DISASSEMBLY



Idle Speed Solenoid Removal

Remove the attaching screws, then remove the Idle Speed Solenoid. The Idle Speed Solenoid should not be immersed in any carburetor cleaner, and should always be removed before complete carburetor overhaul, as carburetor cleaner will damage the internal components.

Idle Mixture Needle Plug Removal
  1. Use hacksaw to make two parallel cuts in the throttle body, one on each side of the locator points near one idle mixture needle plug. The distance between the cuts will depend on the size of the punch to be used. Cuts should reach down to the steel plug, but should but extend more than 1 / 8 in. (3mm) beyond the locator points.
  2.  
  3. Place a flat punch at a point near the ends of the saw marks in the throttle body. Hold the punch at a 45° angle, and drive it into the throttle body until the casting breaks away, exposing the hardened steel plug. The plug will break, rather than remaining intact. Remove all the loose pieces.
  4.  
  5. Repeat the procedure for the other idle mixture needle plug.
  6.  

Idle Air Bleed Valve Removal
  1. Cover internal bowl vents and air inlets to the bleed valve with masking tape.
  2.  
  3. Carefully align a 7 / 64 in. drill bit on rivet head. Drill only enough to remove head of each rivet holding the idle air bleed valve cover.
  4.  
  5. Use a suitably sized punch to drive out the remainder of the rivet from the castings. Repeat procedure with other rivet.
  6.  


CAUTION
For the next operation, safety glasses must be worn to protect eyes from possible metal shaving damage.

  1. Lift off cover and remove any pieces of rivet still inside tower. Use shop air to blow out any remaining chips.
  2.  
  3. Remove idle air bleed valve from the air horn.
  4.  
  5. Remove and discard O-ring seals from valve. New O-ring seals are required for reassembly. The idle air bleed valve is serviced as a complete assembly only.
  6.  

Air Horn Removal
  1. Remove upper choke lever from the end of choke shaft by removing retaining screw. Rotate upper choke lever to remove choke rod from slot in lever.
  2.  
  3. Remove choke rod from lower lever inside the float bowl casting. Remove rod by holding lower lever outward with small screwdriver and twisting rod counterclockwise.
  4.  
  5. Remove secondary metering rods by removing the small screw in the top of the metering rod hanger. Lift upward on the metering rod hanger until the secondary metering rods are completely out of the air horn. Metering rods may be disassembled from the hanger by rotating the ends out of the holes in the end of the hanger.
  6.  
  7. Remove pump link retainer and remove link from pump lever.
  8.  

Do not attempt to remove the lever, as damage to the air horn could result.

  1. Remove front vacuum break hose from tube on float bowl.
  2.  
  3. Remove 11 air horn-to-bowl screws; then remove the 2 countersunk attaching screws located next to the venturi. If used, remove secondary air baffle deflector from beneath the 2 center air horn screws.
  4.  
  5. Remove air horn from float bowl by lifting it straight up. The air horn gasket should remain on the float bowl for removal later.
  6.  

When removing air horn from float bowl, use care to prevent damaging the mixture control solenoid connector, Throttle Position Sensor (TPS) adjustment lever, and the small tubes protruding from the air horn. These tubes are permanently pressed into the air horn casting. DO NOT remove them.

  1. Remove front vacuum break bracket attaching screws. The vacuum break assembly may now be removed from the air valve dashpot rod, and the dashpot rod from the air valve lever.
  2.  

Do not place vacuum break assembly in carburetor cleaner, as damage to vacuum break will occur.

  1. Remove Throttle Position Sensor (TPS) plunger by pushing plunger down through seal in air horn.
  2.  
  3. Remove the throttle position solenoid seal and pump plunger stem seal by inverting air horn and using a small screwdriver to remove staking holding seal retainers in place. Remove and discard retainers and seals.
  4.  

Use care in removing the throttle position solenoid plunger seal retainer and pump plunger stem seal retainer to prevent damage to air horn casting. New seals and retainers are required for reassembly.

  1. Invert air horn, and use Tool J-28696-4, BT-7967A, or equivalent, to remove rich mixture stop screw and spring.
  2.  
  3. Use a suitable punch to drive the lean mixture screw plug and rich mixture stop screw plug out of the air horn. Discard the plugs.
  4.  
  5. Further disassembly of the air horn is not required for cleaning purposes.
  6.  

The choke valve and choke valve screws, the air valves and air valve shaft should not be removed. However, if it is necessary to replace the air valve closing springs or center plastic eccentric cam, a repair kit is available. Instructions for assembly are included in the repair kit.

Float Bowl Disassembly

The following tools, or their equivalents, will be necessary for this procedure: J-28696-10, BT-7928, J-22769, BT-3006M, J-28696-4, and BT-7928

  1. Remove solenoid metering rod plunger by lifting straight up.
  2.  
  3. Remove air horn gasket by lifting it from the dowel locating pins on float bowl. Discard gasket.
  4.  
  5. Remove pump plunger from pump well.
  6.  
  7. Remove staking holding Throttle Position Sensor (TPS) in bowl as follows:
    1. Lay a flat tool or metal piece across bowl casting to protect gasket sealing surface.
    2.  
    3. Use a small screwdriver to depress throttle position solenoid sensor lightly and hold against spring tension.
    4.  
    5. Observing safety precautions, pry upward with a small chisel or equivalent to remove bowl staking, making sure prying force is exerted against the metal piece and not against the bowl casting. Use care not to damage the throttle position solenoid sensor.
    6.  
    7. Push up from bottom on electrical connector and remove throttle position solenoid and connector assembly from bowl. Use care in removing sensor and connector assembly to prevent damage to this critical electrical part.
    8.  
    9. Remove spring from bottom of throttle position solenoid well in float bowl.
    10.  

  8.  
  9. Remove plastic bowl insert from float bowl.
  10.  
  11. Carefully lift each metering rod out of the guided metering jet, checking to be sure the return spring is removed with each metering rod.
  12.  

Use extreme care when handling these critical parts to avoid damage to the metering rod and spring.

  1. Remove the mixture control solenoid from the float bowl as follows:
    1. Remove screw attaching solenoid connector to float bowl. Do not remove solenoid connector from float bowl until called for in text.
    2.  
    3. Use Tool J-28696-10, BT-7928, or equivalent, to remove lean mixture (solenoid) screw. Do not remove plunger return spring or connector and wires from the solenoid body. The mixture control solenoid, with plunger and connector, is only serviced as a complete assembly.
    4.  
    5. Remove rubber gasket from top of solenoid connector and discard.
    6.  
    7. Remove solenoid screw tension spring (next to float hanger pin).
    8.  

  2.  
  3. Remove float assembly and float needle by pulling up on retaining pin. Remove needle and seat and gasket using set remover Tool J-22769, BT-3006M, or equivalent.
  4.  
  5. Remove large mixture control solenoid tension spring from boss on bottom of float bowl located between guided metering jets.
  6.  
  7. If necessary, remove the primary main metering jets using special Tool J-28696-4, BT-7928, or equivalent.
  8.  

Use care installing tool on jet, to prevent damage to the metering rod guide (upper area), and locating tool over vertical float sections on lower area of jet. Also, no attempt should be made to remove the secondary metering jets (metering orifice plates). These jets are fixed and, if damaged, entire bowl replacement is required.

  1. Remove pump discharge check ball retainer and turn bowl upside down, catching discharge ball as if falls.
  2.  
  3. Remove secondary air baffle, if replaced is required.
  4.  
  5. Remove pump well fill slot baffle only if necessary.
  6.  

Choke Disassembly

Tools J-9789-118, BT-30-15, or their equivalents, will be necessary for this procedure.

The tamper-resistant choke cover is used to discourage unnecessary readjustment of the choke thermostatic cover and coil assembly. However, if it is necessary to remove the cover and coil assembly during normal carburetor disassembly for cleaning and normal carburetor disassembly for cleaning and overhaul, the procedures below should be followed.

  1. Support float bowl and throttle body, as an assembly, on a suitable holding fixture such as Tool J-9789-118, BT-30-15, or equivalent.
  2.  

  1. Carefully align a 5 / 32 in. drill bit on the rivet head and drill only enough to remove the rivet head. Drill the 2 remaining rivet heads, then use a drift and small hammer to drive the remainder of the rivets out of the choke housing.
  2.  

Use care in drilling to prevent damage to the choke cover or housing.

  1. Remove the 2 conventional retainers, retainer with tab, and choke cover assembly from choke housing.
  2.  
  3. Remove choke housing assembly from float bowl by removing retaining screw and washer inside the choke housing. The complete choke assembly can be removed from the float bowl by sliding outward.
  4.  
  5. Remove secondary throttle valve lock-out lever from float bowl.
  6.  
  7. Remove lower choke lever from inside float bowl cavity by inverting bowl.
  8.  
  9. To disassemble intermediate choke shaft from choke housing, remove coil lever retaining screw at end of shaft inside the choke housing. Remove thermostatic coil lever from flats on intermediate choke shaft.
  10.  
  11. Remove intermediate choke shaft from the choke housing by sliding it outward. The fast idle cam can now be removed from the intermediate choke shaft. Remove the cup seal from the float bowl for cleaning purposes. DO NOT ATTEMPT TO REMOVE THE INSERT!
  12.  
  13. Remove fuel inlet nut, gasket, check valve, filter assembly and spring. Discard check valve filter assembly and gasket.
  14.  
  15. Remove 3 throttle body-to-bowl attaching screws and lockwashers and remove throttle body assembly.
  16.  
  17. Remove throttle body-to-bowl insulator gasket.
  18.  

Throttle Body Disassembly

Tools J-29030-B and BT-7610B, or their equivalents, will be necessary for this procedure.

Place throttle body assembly on carburetor holding fixture to avoid damage to throttle valves.

  1. Remove pump rod from the throttle lever by rotating the rod until the tang on the rod aligns with the slot in the lever.
  2.  
  3. Use Tool J-29030-B, BT-7610B, or equivalent, to remove idle mixture needles for thorough throttle body cleaning.
  4.  
  5. Further disassembly of the throttle body is not required for cleaning purposes. The throttle valve screws are permanently staked in place and should not be removed. The throttle body is serviced as a complete assembly.
  6.  

ASSEMBLY



The following tools, or their equivalents, will be necessary for this procedure: J-29030-B, BT-7610B, J-9789-118, BT-30-15, J-23417, BT-6911, J-28696-4, J-22769, BT-3006M, J-33815-1, BT-8253-A, J-28696-10, and BT-7928.

  1. Install the lower end of the pump rod in the throttle lever by aligning the tang on the rod with the slot in the lever. The end of the rod should point outward toward the throttle lever.
  2.  
  3. Install idle mixture needles and springs using Tool J-29030-B, BT-7610B, or equivalent. Lightly seat each needle and then turn counterclockwise 3 turns, the final idle mixture adjustment is made on the vehicle.
  4.  
  5. If a new float bowl assembly is used, stamp or engrave the model number on the new float bowl. Install new throttle body-to-bowl insulator gasket over 2 locating dowels on bowl.
  6.  
  7. Install throttle body making certain throttle body is properly located over dowels on float bowl. Install 3 throttle body-to-bowl screws and lockwashers and tighten evenly and securely.
  8.  
  9. Place carburetor on proper holding fixture such as J-9789-118, BT-30-15 or equivalent.
  10.  
  11. Install fuel inlet filter spring, a new check valve filter assembly, new gasket and inlet nut. Tighten nut to 18 ft. lbs. (24 Nm).
  12.  

When installing a service replacement filter, make sure the filter is the type that includes the check valve to meet government safety standard. New service replacement filters with check valve meet this requirement. When properly installed, the hole in the filter faces toward the inlet nut. Ribs on the closed end of the filter element prevent it from being installed incorrectly, unless forced. Tightening beyond the specified torque, 18 ft. lbs. (24 Nm), can damage the nylon gasket.

  1. Install a new cup seal into the insert on the side of the float bowl for the intermediate choke shaft. The lip on the cup seal faces outward.
  2.  
  3. Install the secondary throttle valve lock-out lever on the boss of the float bowl, with the recess hole in the lever facing inward.
  4.  
  5. Install the fast idle cam on the intermediate choke shaft (steps on cam face downward).
  6.  
  7. Carefully install fast idle cam and intermediate choke shaft assembly in the choke housing. Install the thermostatic coil lever on the flats on the intermediate choke shaft. Inside thermostatic choke coil lever is properly aligned when both inside and outside levers face toward the fuel inlet. Install inside lever retaining screw into the end of the intermediate choke shaft.
  8.  
  9. Install lower choke rod (inner) lever into cavity in float bowl.
  10.  
  11. Install choke housing to bowl, sliding intermediate choke shaft into lower (inner) lever. Tool J-23417, BT-6911 or equivalent, can be used to hold the lower choke lever in correct position while installing the choke housing. The intermediate choke shaft lever and fast idle cam are in correct position when the tang on lever is beneath the fast idle cam.
  12.  
  13. Install choke housing retaining screws and washers. Check linkage for freedom of movement. Do not install choke cover and coil assembly until inside coil lever is adjusted.
  14.  
  15. If removed, install air baffle in secondary side of float bowl with notches toward the top. Top edge of baffle must be flush with bowl casting.
  16.  
  17. If removed, install baffle inside of the pump well with slot toward the bottom.
  18.  
  19. Install pump discharge check ball and retainer screw in the passage next to the pump well.
  20.  
  21. If removed, carefully install primary main metering jets in bottom of float bowl using or Tool J-28696-4, BT-7928, equivalent.
  22.  

Use care in installing jets to prevent damage to metering rod guide.

  1. Install large mixture control solenoid tension spring over boss on bottom of float bowl.
  2.  
  3. Install needle seat assembly, with gasket, using seat installer J-22769, BT-3006M, or equivalent.
  4.  
  5. To make adjustment easier, carefully bend float arm before assembly.
  6.  
  7. Install float needle onto float arm by sliding float lever under needle pull clip. Proper installation of the needle pull clip is to hook the clip over the edge of the float on the float arm facing the float pontoon.
  8.  
  9. Install float hinge pin into float arm with end of loop of pin facing pump well. Install float assembly by aligning needle in the seat, and float hinge pin into locating channels in float bowl. DO NOT install float needle pull clip into holes in float arm.
  10.  
  11. Make a float level adjustment as necessary.
  12.  
  13. Install mixture control solenoid screw tension spring between raised bosses next to float hanger pin.
  14.  
  15. Install mixture control solenoid and connector assembly as follows:
    1. Install new rubber gasket on top of solenoid connector.
    2.  
    3. Install solenoid carefully in the float chamber, aligning pin on end of solenoid with hole in raised boss at bottom of bowl. Align solenoid connector wires to fit in slot in bowl.
    4.  
    5. Install lean mixture (solenoid) screw through hole in solenoid bracket and tension spring in bowl, engaging first 6 screw threads to assure proper thread engagement.
    6.  
    7. Install mixture control solenoid gauging Tool J-33815-1, BT-8253-A, or equivalent over the throttle side metering jet rod guide, and temporarily install solenoid plunger.
    8.  
    9. Holding the solenoid plunger against the solenoid stop, use Tool J-28696-10, BT-7928, or equivalent, to turn the lean mixture (solenoid) screw slowly clockwise, until the solenoid plunger just contacts the gauging tool. The adjustment is correct when the solenoid plunger is contacting BOTH the solenoid stop and the gauging tool.
    10.  
    11. Remove solenoid plunger and gauging tool.
    12.  

  16.  
  17. Install connector attaching screw, but DO NOT overtighten, as that could cause damage to the connector.
  18.  
  19. Install throttle position sensor return spring in bottom of well in float bowl.
  20.  
  21. Install throttle position sensor and connector assembly in float bowl by aligning groove in electrical connector with slot in float bowl casting. Push down on connector and sensor assembly so that connector and wires are located below bowl casting surface.
  22.  
  23. Install plastic bowl insert over float valve, pressing downward until properly seated (flush with bowl casting surface).
  24.  
  25. Slide metering rod return spring over metering rod tip until small end of spring stops against shoulder on rod. Carefully install metering rod and spring assembly through holding in plastic bowl insert and gently lower the metering rod into the guided metering jet, until large end of spring seats on the recess on end of jet guide.
  26.  


CAUTION
Do not force metering rod down in jet. Use extreme care when handling these critical parts to avoid damage to rod and spring. If service replacement metering rods, springs and jets are installed, they must be installed in matched sets.

  1. Install pump return spring in pump well.
  2.  
  3. Install pump plunger assembly in pump well.
  4.  
  5. Holding down on pump plunger assembly against return spring tension, install air horn gasket by aligning pump plunger stem with hole in gasket, and aligning holes in gasket over throttle position solenoid plunger, solenoid plunger return spring metering rods, solenoid attaching screw and electrical connector. Position gasket over the two dowel locating pins on the float bowl.
  6.  
  7. Holding down on air horn gasket and pump plunger assembly, install the solenoid-metering rod plunger in the solenoid, aligning slot in end of plunger with solenoid attaching screw. Be sure plunger arms engage top of each metering rod plunger.
  8.  
  9. If a service replacement mixture control solenoid package is installed, the solenoid and plunger MUST be installed as a matched set.
  10.  

Air Horn Assembly

The following tools, or their equivalents, will be necessary for this procedure: J-28696-10, J-2869-4, BT-7967A, J-34935-1, BT-8420A, BT-7928, J-33815-2, and BT-8353B.

  1. If removed, install the throttle position solenoid adjustment screw in the air horn using Tool J-28696-10, BT-7967A, or equivalent. Final adjustment of the throttle position sensor is made on the vehicle.
  2.  
  3. Inspect the air valve shaft pin for lubrication. Apply a liberal quantity of lithium base grease to the air valve shaft pin, especially in the area contacted by the air valve spring.
  4.  
  5. Install new pump plunger and throttle position solenoid plunger seals and retainers in air horn casting. The lip on the seal faces outward, away from the air horn mounting surface. Lightly stake seal retainer in three places, choosing locations different from the original stakings.
  6.  
  7. Install rich mixture stop screw and rich authority adjusting spring from bottom side of the air horn. Use Tool J-2869-4, BT-7967A, or equivalent, to bottom the stop screw lightly, then back out 1 / 4 turn. Final adjustment procedure will be covered later in this section.
  8.  
  9. Install throttle position solenoid actuator plunger in the seal.
  10.  
  11. Carefully lower the air horn assembly onto the float bowl while positioning the throttle position solenoid adjustment lever over the throttle position solenoid sensor and guiding pump plunger stem through the seal in the air horn casting. To ease installation, insert a thin screwdriver between the air horn gasket and float bowl to raise the throttle position solenoid adjustment lever, positioning it over the throttle position solenoid sensor.
  12.  
  13. Make sure that the bleed tubes and accelerating well tubes are positioned properly through the holes in the air horn gasket. Do not force the air horn assembly onto the bowl, but lower it lightly into place over the 2 dowel locating pins.
  14.  
  15. Install 2 long air horn screws and lockwashers, 9 short screws and lockwashers and 2 countersunk screws located next to the carburetor venturi area. Install secondary air baffle beneath the No. 3 and 4 screws. Tighten all screws evenly and securely.
  16.  
  17. Install air valve rod into slot in the lever on the end of the air valve shaft. Install the other end of the rod in hole in front vacuum break plunger. Install front vacuum break and bracket assembly on the air horn, using 2 attaching screws. Tighten screws securely. Connect pump link to pump lever and install retainer.
  18.  

Use care installing the roll pin to prevent damage to the pump lever bearing surface and casting bosses.

  1. Install 2 secondary metering rods into the secondary metering rod hanger (upper end of rods point toward each other). Install secondary metering rod holder, with rods, onto air valve cam follower. Install retaining screw and tighten securely. Work air valves up and down several times to make sure they remove freely in both directions.
  2.  
  3. Connect choke rod into lower choke lever inside bowl cavity. Install choke rod in slot in upper choke lever, and position lever on end of choke shaft, making sure flats on end of shaft align with flats in lever. Install attaching screw and tighten securely. When properly installed, the number on the lever will face outward.
  4.  
  5. Adjust the rich mixture stop screw:
    1. Insert external float gauging Tool J-34935-1, BT-8420A, or equivalent, in the vertical D-shaped vent hole in the air horn casting (next to the idle air bleed valve) and allow it to float freely.
    2.  
    3. Read (at eye level) the mark on the gauge, in inches, that lines up with the tip of the air horn casting.
    4.  
    5. Lightly press down on gauge, and again read and record the mark on the gauge that lines up with the top of the air horn casting.
    6.  
    7. Subtract gauge UP dimension, found in Step b, from gauge DOWN dimension, found in Step c, and record the difference in fractions of an inch. This difference in dimension is the total solenoid plunger travel.
    8.  
    9. Insert Tool J-28696-10, BT-7928, or equivalent, in the access hole in the air horn, and adjust the rich mixture stop screw to obtain 1 / 8 in. (3mm) total solenoid plunger travel.
    10.  

  6.  
  7. With the solenoid plunger travel correctly set, install the plugs supplied in the service kit into the air horn to retain the setting and prevent fuel vapor loss:
    1. Install the plug, hollow end down, into the access hole to the lean mixture (solenoid) screw and use a suitably sized punch to drive the plug into the air horn until top of plug is even with the lower edge of the hole chamber.
    2.  
    3. In a similar manner, install the plug over the rich mixture screw access hole and drive the plug into place so that the tip of the plug is 1 / 16 in. (1.5mm) below the surface of the air horn casting.
    4.  

  8.  
  9. Install the Idle Air Bleed Valve as follows:
    1. Lightly coat 2 new O-ring seals with automatic transmission fluid, to aid in their installation on the idle air bleed valve body. The thick seal goes in the upper groove and the thin seal goes in the lower groove.
    2.  
    3. Install the idle air bleed valve in the air horn, making sure that there is proper thread engagement.
    4.  
    5. Insert idle air bleed valve gauging Tool J-33815-2, BT-8353B, or equivalent, in throttle side D-shaped vent hole of the air horn casting. The upper end of the tool should be positioned over the open cavity next to the idle air bleed valve.
    6.  
    7. Hold the gauging tool down lightly so that the solenoid plunger is against the solenoid stop, then adjust the idle air bleed valve so that the gauging tool will pivot over and just contact the top of the valve.
    8.  
    9. Remove the gauging tool.
    10.  
    11. The final adjustment of the idle air bleed valve is made on the vehicle to obtain idle mixture control.
    12.  

  10.  
  11. Perform the Air Valve Spring Adjustment and Choke coil Lever Adjustment as previously described.
  12.  
  13. Install the cover and coil assembly in the choke housing, as follows:
    1. Place the cam follower on the highest step of the fast idle cam.
    2.  
    3. Install the thermostatic cover and coil assembly in the choke housing, making sure the coil tang engages the inside coil pickup lever. Ground contact for the electric choke is provided by a metal plate located at the rear of the choke cover assembly. DO NOT install a choke cover gasket between the electric choke assembly and the choke housing.
    4.  
    5. A choke cover retainer kit is required to attach the choke cover to the choke housing. Follow the instructions found in the kit and install the proper retainer and rivets using a suitable blind rivet tool.
    6.  
    7. It may be necessary to use an adapter (tube) if the installing tool interferes with the electrical connector tower on the choke cover.
    8.  

  14.  
  15. Install the hose on the front vacuum brake and on the tube on the float bowl.
  16.  
  17. Position the idle speed solenoid and bracket assembly on the float bowl, retaining it with 2 large countersunk screws.
  18.  
  19. Perform the Choke Rod-Fast Idle Cam Adjustment, Primary (Front) Vacuum Break Adjustment, Air Valve Rod Adjustment - Front, Unloader Adjustment and the Secondary Lockout Adjustment as previously described.
  20.  
  21. Reinstall the carburetor on the vehicle with a new flange gasket.
  22.  

 
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