Cummins/Onan B43 / B48 — Nikki Carburetor Adjustment
Adjustments
For the Cummins/Onan B43 / B48, from the Cummins/Onan owner's manual.
Applies to: Cummins/Onan B48, Cummins/Onan B43M engine, Cummins/Onan B48M engine
Before you start
- Overtightening the mixture adjustment screw will cause carburetor damage; turn in only until light tension can be felt.
Procedure
- Identify whether the carburetor is the full adjustment or limited idle mixture type.
- Full adjustment: back idle mixture screw out 3/4 turn; back main mixture screw out 1-1/4 to 1-1/2 turns.
- Limited idle mixture: idle fuel adjustment is restricted to 1/4 turn by a limiting cap; no main fuel adjustment screw.
- Start the engine and allow it to warm up thoroughly (at least 10 minutes).
- Move the engine speed control to the slow position; bend or turn the low speed stop so the throttle stop screw controls speed.
- Adjust the throttle stop screw for 1000 rpm idle.
- Full adjustment: set the idle mixture screw about 1/8 turn outward (rich) from the midpoint of the highest-speed range.
- Readjust the throttle stop screw for 1000 rpm idle, then adjust the governor low speed stop for 1100 rpm idle.
- Move the speed control to fast; bend the high speed stop so the engine runs at the manufacturer's recommended speed.
- Check the main mixture by rapidly accelerating from idle to full; adjust in 1/8 turn increments if needed, up to 1/2 turn beyond original.
Specs & values
- Idle mixture 3/4 turn out (full adjustment)
- Main mixture 1-1/4 to 1-1/2 turns out (full adjustment)
- Throttle stop 1000 rpm idle
- Governor low speed stop 1100 rpm idle
Full service-parts list: Cummins/Onan B48 →
Source: Cummins/Onan owner'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.