Cummins/Onan — Idle Fuel Mixture Adjustment (prior to Spec F)
Adjustments · carburetor idle mixture
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Forcing a mixture adjusting screw in tight will score the needle and seat.
Procedure
- If the carburetor was overhauled, turn idle and main mixture screws in by hand until they seat.
- Gasoline: turn idle mixture screw out 1 turn and main fuel mixture screw out 1-3/8 turns.
- LPG: turn idle mixture screw out 1-1/4 turns and main fuel mixture screw out 2-1/2 turns.
- Start the genset and warm up ten minutes under 1/2 to 3/4 rated load.
- Disconnect the load (check for zero amps).
- Turn the idle mixture screw clockwise until the engine begins to stumble.
- Counting turns, turn it counterclockwise until it stumbles again; set halfway.
- Use a CO meter to adjust to 6-8% CO (gasoline) or 4-6% CO (LPG) for closer adjustment.
Tools required
- CO meter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 6-8% CO (gasoline)
- 4-6% CO (LPG)
Source: Cummins/Onan service 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.