General Motors GMG-857 — Carburetor Air-Gas Valve Inspection and Cleaning
Maintenance · Gas carburetor
For the General Motors GMG-857, from the General Motors operator's manual.
Applies to: KEM GMG-857, General Motors 5.7L V8 (350 CID) engine
Before you start
- Do not attempt to measure gas pressure with a mercury manometer.
Procedure
- Measure gas inlet pressures at idle and full load with a water manometer.
- Remove the air valve cover and lift out the spring and air-gas valve with diaphragm.
- Inspect diaphragm for holes, charring or stiffness.
- Check inside diameter of the air valve cup and fins of the gas jet for wear.
- Inspect the inside diameter of the gas jet and O.D. of the small tapered gas valve for wear.
- Wash all parts thoroughly in kerosene or equivalent (water and detergent for digester gas).
- Reassemble cleaned or replaced parts in the bowl, centering the spring on the air valve.
- Replace and fasten the cover; lift valve several times to assure free travel.
Tools required
- water manometer Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Gas pressure at idle should be 5 to 7" depending on air-fuel mixture
- Using 1050 BTU natural gas, 7" pressure produces a straight power mixture, 5" gives economy mixtures at light load
Full service-parts list: KEM GMG-857 →
Source: General Motors operator's manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.