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HOMESERVICE MANUALSGMC1977FORWARD CONTROL 7.4L ENG VIN Y, AUTOMATIC TRANS, TRANS MFR CD 475/M40REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETUNE-UPTUNE-UP PROCEDURES - V8IDLE MIXTUREBEST IDLE PROCEDURE1979 HEAVY DUTY MODELS
1977 GMC Forward Control 7.4L Eng VIN Y, Automatic Trans, Trans Mfr CD 475/M40

1979 Heavy Duty Models

1977 GMC Forward Control 7.4L Eng VIN Y, Automatic Trans, Trans Mfr CD 475/M40SECTION 1979 Heavy Duty Models
  1. Set parking brake and block drive wheels. Remove air cleaner after engine reaches normal operating temperature. Place shift lever in Neutral, and connect a tachometer to engine.
  2. As a starting point, turn idle mixture screws in to lightly seat and then back out 2 turns. Do not turn screw tightly against seat or damage may result.
    1978 PROPANE ENRICHED RPM SPECIFICATIONS

    Application RPM
    305"
      Man. Trans. 600-640
      Auto. Trans. 500-535
    350"
      Federal
        Man. Trans. 700-725
        Auto. Trans. 500-550
      California
        Man. Trans. 700-720
        Auto. Trans. 500-550
      High Altitude 500-550
    400" (1)
    454" 550-580
    (1) See Emission Control Tune-Up Decal in engine compartment.
    1979 PROPANE ENRICHED RPM SPECIFICATIONS

    Application RPM
    305"
      Man. Trans. 770-800
      Auto. Trans. 540-560
    350"
      Federal
        Man. Trans. 870-900
        Auto. Trans.
          With Chassis Cab 560-580
          All Others 540-560
      California Man. Trans. 700-720
        Auto. Trans. 500-520
        High Altitude 530-550
    400"
      Federal
        Auto. Trans. 580-600
      California
        Auto. Trans. 540-560
    454"
      Man. Trans. 750
      Auto. Trans. 580
  3. With engine running, choke wide open and transmission in Neutral, adjust idle speed to specifications. Adjust idle mixture screw to obtain highest RPM.
  4. Readjust idle speed screw to specifications and readjust mixture screw to obtain highest RPM. Shut down engine, remove gauges and install air cleaner.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.