Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1985STEP VAN P20, 4.8 T, AUTOMATIC, 475/M40REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMCARBURETOR - ROCHESTER M4M SERIES 4-BBLOVERHAULDISASSEMBLYAIR HORN
1985 Chevrolet Step Van P20, 4.8 T, Automatic, 475/M40
Air Horn
1985 Chevrolet Step Van P20, 4.8 T, Automatic, 475/M40SECTION Air Horn
- 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.
- Remove retaining screw from secondary metering rod hanger. Lift out hanger and secondary metering rods as an assembly.
- Drive pump lever pivot pin inward until pump lever can be removed. Disconnect pump rod from pump lever, noting location of rod in lever for reassembly reference.
- Remove air horn-to-float bowl 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 for later removal.
- Invert air horn to remove pump plunger stem seal (if used). 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.
- Further disassembly of air horn is not required. If air valve closing spring and/or plastic cam need replacing, a repair kit is available.
CAUTION:
On M4MEF models, DO NOT
turn or remove Rich Stop Adjusting Bushing. Unnecessary adjustment of this bushing could result in engine damage or increased exhaust emissions.
RENDER: 1.0x
NO RELATED
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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.