General Motors — Servicing the Impco gas carburetor
Maintenance
From a General Motors operator's manual covering: 4.3L 262 CID, 4.3L V6, GMG-643. Check it applies to your exact model before use.
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 (not a mercury manometer).
- If conditions indicate carburetor malfunction remove the air valve cover and lift out the spring and air-gas valve complete with diaphragm.
- Inspect the diaphragm for holes or charring; it should be reasonably flexible.
- If diaphragms prove sound check the inside diameter of the air valve cup and the fins on the outside of the gas jet for grooves.
- Inspect the inside diameter of the gas jet and the O.D. of the small tapered gas valve for wear.
- Wash all parts thoroughly in kerosene or the equivalent; if the fuel is digester (sewage) gas wash parts in water and detergent.
- Reassemble cleaned or replaced parts centering the spring on the air valve and replace and fasten the cover.
Tools required
- Water manometer Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- With 1050 BTU natural gas 7 in. pressure produces a straight power mixture; 5 in. pressure gives economy mixtures at light load
- Gas pressure may be raised to 12 to 16 in. of water column if necessary
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.