Cummins/Onan B43 / B48 — Carburetor Adjustment
Adjustments
For the Cummins/Onan B43 / B48, from the Cummins/Onan service manual.
Applies to: Cummins/Onan B48
Before you start
- Overtightening the mixture adjustment screw will cause carburetor damage. Turn mixture adjustment screw in only until light tension can be felt.
Procedure
- Initial settings: Marvel Schebler - idle mixture screw out 1 turn, main mixture screw out 1-1/4 turns.
- Walbro - idle mixture screw out 1-1/8 turns; main mixture screw out 1-1/2 turns.
- Nikki - idle mixture screw out 3/4 turn; main mixture screw out 1-1/4 to 1-1/2 turns.
- Nikki with semi-automatic choke and limited idle adjustment - no initial adjustments; when replacing idle screw turn out 1-1/4 turns.
- Start engine and allow it to warm up thoroughly (at least 10 minutes).
- Move engine speed control to slow position and adjust throttle stop screw to obtain specified rpm.
- Determine best idle mixture setting; set about 1/8 turn outward (rich) from midpoint.
- Readjust throttle stop screw and release governor arm.
- Adjust governor low speed stop for 1100 +/- 100 rpm idle.
- Move speed control to fast position and bend high speed stop to equipment manufacturer's recommended speed.
- Check main mixture adjustment by rapidly accelerating from idle to full speed; adjust in 1/8 turn increments if needed.
Tools required
- screwdriver Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Marvel Schebler low idle 1200 rpm (1100 rpm with governor low speed adjustment screw)
- Walbro and Nikki low idle 1000 rpm
- Governor low speed idle 1100 +/- 100 rpm
Full service-parts list: Cummins/Onan B48 →
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.