GM Metro/Sprint 1985-1993 Repair Guide

Carburetor

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This 2-barrel, downdraft type carburetor has primary and secondary systems. The primary system operates under normal driving conditions and the secondary system operates under high speed-high load driving conditions. The choke valve is provided in the primary system.

The main components and their functions are as follows:



The primary system has a mixture control solenoid valve which is operated by the electrical signals from the Electronic Control Module (ECM). The solenoid maintains the optimum air/fuel ratio of the primary slow and the primary main fuel systems at all times.
 
A fuel cut solenoid valve stops fuel flow under deceleration and prevents dieseling when the engine is stopped.
 
An accelerator pump system provides additional fuel for acceleration.
 
The secondary system has a secondary vacuum break unit which is operated by the vacuum from the primary side and actuates the secondary throttle valve.
 
The choke system is a full automatic type using a thermowax
 
The switch vent solenoid valve provided on top of the float chamber is to reduce evaporative emissions.
 

ADJUSTMENTS



See Figures 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10



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Fig. Fig. 1: Accelerator cable free play adjustment



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Fig. Fig. 2: Checking fuel level



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Fig. Fig. 3: Idle up adjustment

Accelerator Cable
  1. Check accelerator cable for play. Move the accelerator cable up and down to determine free-play.
  2.  



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Fig. Fig. 4: Checking choke valve to carburetor valve clearance



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Fig. Fig. 5: Checking choke piston



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Fig. Fig. 6: Adjusting fast idle

  1. Accelerator cable free-play should not exceed 0.40-0.59 in. (10-15mm) when engine and carburetor are cold or 0.12-0.19 in. (3-5mm) when engine and carburetor are warm.
  2.  



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Fig. Fig. 7: Unloader lever arm adjustment



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Fig. Fig. 8: Idle speed adjusting screw



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Fig. Fig. 9: Connecting dwell meter to duty check connector



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Fig. Fig. 10: Mixture adjusting screw

  1. If out of specification loosen locknut and adjust by turning adjusting nut.
  2.  

Float
  1. The fuel level in float chamber should be within round mark at the center of the level gauge.
  2.  
  3. If the fuel level is not found within the round mark check the float level and adjust.
  4.  
  5. Remove the air horn and invert it.
  6.  
  7. Measure the distance between the float and the surface of the choke chamber. the measured distance is the float level. Measurement should be 0.21-0.24 in. (5.3-6.3 mm).
  8.  
  9. If the float level is out of specification, adjust it by bending the tongue up and down. This measurement should be made without a gasket on the air horn.
  10.  

Idle Up Actuator

The idle-up actuator operates even when cooling fan is running. Therefore, idle-up adjustment must be performed when cooling fan is not running.

Manual Transaxle
  1. Warm engine to normal operating temperature.
  2.  
  3. While running engine at idle speed, check to ensure that idle up adjusting screw moves down when lights are turned ON .
  4.  
  5. With lights turned ON check engine rpm. Be sure that heater fan, rear defogger, engine cooling fan and air conditioner are OFF .
  6.  
  7. Engine rpm should be at specified idle speed or 50 rpm higher. If not, adjust using idle up adjusting screw located at top of idle up solenoid.
  8.  
  9. Check idle up system as described above with the heater fan, rear defogger, engine cooling fan and air conditioner are all operated separately.
  10.  

Automatic Transaxle
  1. Warm engine to normal operating temperature.
  2.  
  3. While running engine at idle speed, check to ensure that idle up adjusting screw moves down when brake pedal is depressed and shift selector lever is moved to D range. All accessories should be OFF .
  4.  
  5. Check engine rpm. Engine rpm should be at specified idle speed or 50 rpm higher. If not, adjust using idle up adjusting screw located at top of idle up solenoid.
  6.  

Choke Valve

Perform the following adjustment with the air cleaner top removed and the engine cold.

  1. Check choke valve for smooth movement by pushing it with a finger.
  2.  
  3. Make sure that choke valve is closed almost completely when ambient temperature is below 77°F (25°C) and engine is cold.
  4.  
  5. Check to ensure that choke valve to carburetor bore clearance is 0.004-0.019 in. (0.1-0.5mm) when air temperature is 77°F (25°C) and 0.03-0.06 in. (0.7-1.7mm) when air temperature is 95°F (35°C).
  6.  
  7. If clearance is not within specification, remove the air cleaner case and check strangler spring, choke piston and each link in choke system for smooth operation. Lubricate choke valve shaft and each link with lubricant if necessary. Do not remove riveted choke lever guide.
  8.  
  9. If the clearance is still out of specification, even after lubrication, remove carburetor from intake manifold and remove idle up actuator from carburetor.
  10.  
  11. Turn fast idle cam counterclockwise and insert an available pin into holes on cam and bracket to lock cam. In this state, bend the choke lever up or down with pliers. Bending up causes choke valve to close and vice-versa.
  12.  

Choke Piston
  1. Disconnect choke piston hose at throttle chamber.
  2.  
  3. With choke valve pushed down to its closing side lightly by finger pressure, apply vacuum to choke piston hose and check to ensure that choke valve to carburetor bore clearance is within 0.09-0.10 in. (2.3-2.6mm).
  4.  
  5. With vacuum applied, move choke piston rod toward diaphragm with small screwdriver and check to ensure that choke valve to carburetor bore clearance is within 0.16-0.18 in. (4.0-4.7mm).
  6.  

Fast Idle
  1. Drain the cooling system.
  2.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. After draining, disconnect choke hoses from the carburetor thermo element holder and plug them.
  2.  
  3. Install radiator drain plug and refill cooling system.
  4.  
  5. Start engine and check hoses for leakage. After making sure that both hoses are free from leakage, warm engine to normal operating temperature.
  6.  
  7. With engine running at idle speed, force fast idle cam to rotate counterclockwise with pliers until mark on cam aligns with center of cam follower.
  8.  
  9. Check to ensure that engine speed is within 2100-2700 rpm.
  10.  
  11. If engine speed is not within specification, adjust by turning fast idle adjusting screw.
  12.  
  13. After adjusting fast idle speed, reconnect hoses.
  14.  

Unloader
  1. Perform this adjustment when engine is cold.
  2.  
  3. Remove air cleaner cover.
  4.  
  5. Make sure that choke valve is closed.
  6.  
  7. Fully open throttle valve and check choke valve to carburetor bore clearance to ensure is is within 0.10-0.12 in. (2.5-3.3mm).
  8.  
  9. If clearance is out of specification, adjust by bending the unloader arm.
  10.  

Idle Speed
  1. Check emission control system wires, accelerator cable, ignition timing and valve lash prior to setting idle speed.
  2.  
  3. Connect a tachometer to the engine.
  4.  
  5. Place transaxle in N and set parking brake.
  6.  
  7. Warm engine to normal operating temperature.
  8.  
  9. With lights, heater fan, rear defogger, cooling fan and air conditioner all OFF , check idle speed.
  10.  
  11. If idle speed is not within specification adjust by turning throttle screw.
  12.  
  13. After idle speed adjustment, check idle up system.
  14.  
  15. Stop engine and check accelerator cable to ensure that free-play is within specification.
  16.  

Idle Mixture

The carburetor has been calibrated at the factory and should not normally need adjustment. For this reason, the mixture adjustment should never be changed from the original factory setting. However, if during diagnosis, the check indicates the carburetor to be the cause of a driver performance complaint or emission failure, or the carburetor is overhauled or replaced, the idle mixture can be adjusted using the following procedure.

  1. Check the ignition timing and valve lash. Ensure the idle up actuator does not operate and the cooling fan does not run.
  2.  
  3. Remove the carburetor from the intake manifold following normal service procedure to again access to the mixture adjust screw pin covering the mixture adjust screw.
  4.  
  5. Using a 4-4.5mm drill, drill a hole in pin. After drilling, remove pin by punch or equivalent.
  6.  
  7. Reinstall the carburetor and air cleaner following normal service procedures. Connect emission control system hoses and lead wires. Make specified play on accelerator cable and refill cooling system.
  8.  
  9. Place transaxle gear shift lever in N and set parking brake.
  10.  
  11. Start engine and allow it to reach operating temperature.
  12.  
  13. Remove coupler on duty check connector in engine compartment and connect a dwell meter to the connector. Connect the positive terminal of dwell meter to Blue/Red wire and the negative terminal to Black/Green wire.
  14.  
  15. Set dwell meter to 6-cylinder position.
  16.  
  17. Check idle speed and adjust if not within specification.
  18.  
  19. Run engine at idle speed and adjust mixture screw slowly. Adjust until the desired 21-27 degrees is obtained. Allow time for dwell to stabilize after turning screw.
  20.  
  21. Check idle speed after setting mixture and adjust as required.
  22.  
  23. If adjustment cannot be made because the dwell meter does not deflect, check the feedback system.
  24.  

REMOVAL & INSTALLATION



  1. Disconnect the negative battery cable.
  2.  
  3. Drain the cooling system.
  4.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Remove the air cleaner element.
  2.  
  3. Disconnect warm air hose, cool air hose, second air hose, vacuum hose and EGR modulator from air cleaner case.
  4.  
  5. Remove air cleaner, accelerator cable, electric lead wires, emission control hoses, choke hoses and fuel hose.
  6.  
  7. Remove carburetor from intake manifold.
  8.  

To install:
  1. Using a new gasket, install carburetor on intake manifold.
  2.  
  3. Install air cleaner, accelerator cable, electric lead wires, emission control hoses, choke hoses and fuel hose.
  4.  
  5. Connect warm air hose, cool air hose, second air hose, vacuum hose and EGR modulator on air cleaner case.
  6.  
  7. Install the air cleaner element.
  8.  
  9. Refill the cooling system.
  10.  
  11. Connect the negative battery cable.
  12.  

OVERHAUL



See Figure 11

Disassembly
  1. Remove air horn from float chamber after disconnecting hoses and pump lever and removing screws from air horn.
  2.  
  3. Remove float and needle valve from air horn and then needle valve seat and filter.
  4.  
  5. Remove micro switch bracket after removing springs and set screws. Do not remove micro switches from bracket.
  6.  



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Fig. Fig. 11: MR08 carburetor - exploded view

  1. Remove lockplate® and drain plugs and then primary and secondary main jets in float chamber through plug holes using a screwdriver.
  2.  
  3. Remove throttle chamber from float chamber after removing screws.
  4.  

Cleaning
  1. Wash all removable air jets and fuel jets, needle valve, valve seat, filter and float in carburetor cleaner and then dry with compressed air.
  2.  
  3. Blow compressed air into all passages to clean.
  4.  
  5. Clean bottom of float chamber.
  6.  
  7. Do not clean the following parts: micro switches, switch vent solenoid, fuel cut solenoid, mixture control solenoid valve, accelerator pump piston, secondary diaphragm, choke piston, idle up actuator, rubber parts and gaskets or thermo wax element.
  8.  
  9. Do not put drills or wires into fuel passages and metering jets for cleaning. It may cause damage in passages and jets.
  10.  
  11. Do not remove the mixture adjust screw pin located in front of mixture adjust screw.
  12.  

Inspection
  1. Check choke valve and throttle valve for smooth operation.
  2.  
  3. Check rubber cup of pump piston and seal of mixture control solenoid valve for deterioration and damage.
  4.  
  5. Check needle valve and valve seat for wear.
  6.  

Assembly
  1. Install the gasket on the float chamber.
  2.  
  3. After attaching both ends of throttle lever return spring, install throttle chamber to float chamber. Hook throttle valve side end of return spring over the boss on float chamber.
  4.  
  5. Two of the four washers for the throttle chamber securing screws are toothed washers and the other two are spring washers. Install each washer to the throttle chamber and tighten to 3-5 ft. lbs. (4-7 Nm).
  6.  
  7. Install the primary and secondary main jets.
  8.  
  9. Install the gaskets and drain plugs, and then lock plate after installing main jets.
  10.  
  11. Install level gauge seal and level gauge. When installing level gauge, direct its face with small round mark in its center toward float chamber.
  12.  
  13. Install micro switch bracket with micro switches and springs. Do not remove the micro switches from the bracket.
  14.  

When bracket with micro switches has been removed from float chamber for any service and reinstalled after service work, make sure to check switches for operation and adjust if necessary.

  1. Install jets, air bleeders and plug to float chamber.
  2.  
  3. Install balls, injector spring and weights to accelerator pump. Direct "U" bend end side of piston return spring downward.
  4.  
  5. Install needle valve filter, valve seat, gasket, needle valve and float to air horn. After installing float, check for float level and adjust.
  6.  
  7. Install primary and secondary slow air bleeders to air horn.
  8.  
  9. Be sure to silicone grease to mixture control solenoid valve seal before installing air horn to float chamber.
  10.  
  11. Install air horn gasket to float chamber. Use new gasket.
  12.  
  13. Install air horn to float chamber. After installing accelerator pump lever to air horn, check the lever for smooth operation.
  14.  
  15. Connect hoses to carburetor.
  16.  

 
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