Cummins/Onan B43 / B48 — Marvel Schebler 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
- Turn both mixture screws in until lightly seated, then back the idle mixture screw out 1-1/8 turns and the main mixture screw out 1-1/4 turns.
- Start the engine and allow it to warm up thoroughly (at least 10 minutes).
- Move the engine speed control to the slow position.
- Determine if the engine has a governor low speed adjustment screw.
- Pull the governor back so the throttle stop screw is against its stop and hold the governor arm there.
- Adjust the throttle stop screw to obtain 1200 RPM without a low speed screw, or 1100 RPM with one.
- Turn the idle adjustment screw in until speed drops, then out until speed drops again; set about 1/8 turn outward from the midpoint.
- Engines with a governor low speed adjustment screw: adjust it to obtain 1200 RPM.
- Check main mixture by rapidly accelerating from idle to full; turn main screw out in 1/8 turn increments if needed, up to 1/2 turn beyond original.
Specs & values
- Idle mixture 1-1/8 turns out
- Main mixture 1-1/4 turns out
- Throttle stop 1200 rpm (no low speed screw) or 1100 rpm (with screw)
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.