Cummins/Onan — Adjusting the Carburetor
Adjustments
From a Cummins/Onan service manual covering: 0.6MAJ, 1.0MAJ, 1.5MAJ, 1.5MAJ-224R, 1.5MAJ-232R, 2.5MAJ. Check it applies to your exact model before use.
Applies to: Cummins/Onan 2.5MAJ
Procedure
- Turn adjusting screws in gently to seats. Do not force them.
- Back main screw out about 2-1/2 full turns (1-1/2 on downdraft carburetor).
- Start set and allow it to thoroughly warm up under full-load condition.
- Slowly turn main adjusting screw inward (clockwise) for leaner mixture until set loses speed or voltage drops.
- Turn screw outward (counterclockwise) until set carries full load.
- Check operation at various loads.
- If hunting, turn adjusting screw out for richer mixture, not more than 1/2 turn beyond maximum generator output.
- For AC sets, adjust idle screw with no AC load connected.
- For battery charging sets, adjust idle screw at lowest possible charge rate.
- Slowly turn idle adjusting screw inward until set loses speed; then outward until set runs smoothly.
- Adjust throttle idle stop screw to clear throttle shaft stop by 1/32 inch.
Specs & values
- 1/32 inch throttle idle stop clearance
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.