Cummins/Onan — Carburetor Final Adjustment (gasoline)
Adjustments
From a Cummins/Onan service manual covering: 2.5A, 2.5L, 2.7 AJ, 4.0CCK, 6.0CCKB, 6.5 NH, JC - 3CR / 96 T. Check it applies to your exact model before use.
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Before you start
- Valves may be damaged by turning them in too far.
Procedure
- Turn main fuel valve in until engine misses (lean), then out past smooth until uneven (rich); set to mid-point.
- Hold engine at idle and set low speed adjustment screw until a fast idle of 1200 rpm is obtained.
- Turn idle adjustment valve in (lean) and out (rich) until engine idles smoothly.
- Reset low speed adjustment screw so engine idles at 1200 rpm.
- Release throttle; engine should accelerate without hesitation, readjust main fuel valve if not.
Tools required
- Screwdriver Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Idle speed 1200 rpm
- Do not open main fuel valve more than 1/2 turn beyond maximum power point
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.