Cummins/Onan B43M Engine

⚠ Make sure feeler gauge is clean and free of any grease, oil or dirt.

Tools: Flat feeler gauge, Allen wrench

1. Remove spark plugs.
2. Remove breaker box cover. Rotate crankshaft clockwise (facing flywheel) until points are fully open.
3. Remove condenser (screw A) and detach condenser lead and coil lead (screw B).
4. Remove two Allen screws (C) and lift breaker assembly from engine.
5. Replace condenser and point assembly with new parts and reinstall in reverse order of removal.
6. Rotate crankshaft clockwise by hand until points fully open; set point gap with a clean flat feeler gauge to 0.016 in by adjusting the Allen socket head screw (D).
7. Replace breaker box cover, coil wire, spark plugs, and spark plug cables.

⚠ Overtightening the mixture adjustment screw will cause carburetor damage. Turn mixture adjustment screw in only until light tension can be felt.

Tools: screwdriver

1. Initial settings: Marvel Schebler - idle mixture screw out 1 turn, main mixture screw out 1-1/4 turns.
2. Walbro - idle mixture screw out 1-1/8 turns; main mixture screw out 1-1/2 turns.
3. Nikki - idle mixture screw out 3/4 turn; main mixture screw out 1-1/4 to 1-1/2 turns.
4. Nikki with semi-automatic choke and limited idle adjustment - no initial adjustments; when replacing idle screw turn out 1-1/4 turns.
5. Start engine and allow it to warm up thoroughly (at least 10 minutes).
6. Move engine speed control to slow position and adjust throttle stop screw to obtain specified rpm.
7. Determine best idle mixture setting; set about 1/8 turn outward (rich) from midpoint.
8. Readjust throttle stop screw and release governor arm.
9. Adjust governor low speed stop for 1100 +/- 100 rpm idle.
10. Move speed control to fast position and bend high speed stop to equipment manufacturer's recommended speed.
11. Check main mixture adjustment by rapidly accelerating from idle to full speed; adjust in 1/8 turn increments if needed.

⚠ Over-tightening mixture adjustment screw will cause carburetor damage.

Tools: screwdriver, tachometer

1. Warm engine at least 10 minutes.
2. Set throttle to slow; adjust throttle stop screw for 1000 rpm idle.
3. Turn idle mixture screw in until speed drops, then out until speed drops again.
4. Set screw about 1/8 turn outward from midpoint.
5. Readjust throttle stop for 1000 rpm idle, then governor low speed stop for 1100 rpm idle.
6. Move to fast; adjust high speed stop to recommended speed.
7. Check acceleration; adjust main mixture in 1/8 turn increments if needed.

1. Start engine and allow it to warm up thoroughly (at least 10 minutes).
2. Move engine speed control to the slow position.
3. Bend or turn the low speed stop on the governor so the throttle stop screw on the carburetor controls engine speed.
4. Adjust the throttle stop screw for 1000 rpm idle.
5. Adjust the governor low speed stop for 1100 rpm idle.
6. Move engine speed control to the fast position.
7. Bend the high speed stop on the governor so the engine runs at the vehicle manufacturer's recommended speed.

Tools: feeler gauge, torque wrench

1. Tighten rear bearing end plate to specified torque.
2. Measure crankshaft endplay.
3. If too much endplay, remove rear bearing end plate.
4. Add shim between thrust washer and plate.
5. Reinstall end plate with notches aligned to lock pins.
6. Torque and recheck endplay.

Tools: tachometer

1. Engine speed is controlled by adjusting governor spring tension.
2. On side pull governors the governor spring is factory set in the third hole of the governor arm.
3. On front pull governors the governor spring is factory set in the second hole of the governor arm adapter.
4. To increase sensitivity, move spring loop into a hole closer to the pivot.
5. To decrease sensitivity, move spring loop into a hole farther from the pivot.
6. After sensitivity is set, recheck low speed rpm setting and adjust if necessary.
7. Check governor arm, linkage, throttle shaft and lever for binding or excessive slack.

Tools: tachometer

1. Disconnect linkage (A) from one of holes (C).
2. Push linkage (A) and governor arm (B) as far back (toward carburetor) as they will go.
3. Holding linkage and governor arm toward carburetor, insert end of linkage into whichever hole (C) in governor arm lines up closest; if between two holes, insert in next hole out.
4. Adjust governor link length so that with engine stopped and tension on governor spring, throttle shaft stop almost touches carburetor side stop.

⚠ Overtightening the mixture adjustment screw will cause carburetor damage; turn in only until light tension can be felt.

1. 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.
2. Start the engine and allow it to warm up thoroughly (at least 10 minutes).
3. Move the engine speed control to the slow position.
4. Determine if the engine has a governor low speed adjustment screw.
5. Pull the governor back so the throttle stop screw is against its stop and hold the governor arm there.
6. Adjust the throttle stop screw to obtain 1200 RPM without a low speed screw, or 1100 RPM with one.
7. Turn the idle adjustment screw in until speed drops, then out until speed drops again; set about 1/8 turn outward from the midpoint.
8. Engines with a governor low speed adjustment screw: adjust it to obtain 1200 RPM.
9. 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.

⚠ Overtightening the mixture adjustment screw will cause carburetor damage; turn in only until light tension can be felt.

1. Identify whether the carburetor is the full adjustment or limited idle mixture type.
2. Full adjustment: back idle mixture screw out 3/4 turn; back main mixture screw out 1-1/4 to 1-1/2 turns.
3. Limited idle mixture: idle fuel adjustment is restricted to 1/4 turn by a limiting cap; no main fuel adjustment screw.
4. Start the engine and allow it to warm up thoroughly (at least 10 minutes).
5. Move the engine speed control to the slow position; bend or turn the low speed stop so the throttle stop screw controls speed.
6. Adjust the throttle stop screw for 1000 rpm idle.
7. Full adjustment: set the idle mixture screw about 1/8 turn outward (rich) from the midpoint of the highest-speed range.
8. Readjust the throttle stop screw for 1000 rpm idle, then adjust the governor low speed stop for 1100 rpm idle.
9. Move the speed control to fast; bend the high speed stop so the engine runs at the manufacturer's recommended speed.
10. 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.

Tools: magnet tool, parts cleaning solvent

1. Remove the 3/8 x 24 x 7/8 cap screw located behind gear cover and under governor arm.
2. Remove spring and plunger with a magnet tool.
3. Check spring and plunger per measurements: plunger diameter 0.3105 to 0.3125 in (7.89 to 7.94 mm); spring free length 1.00 inch (25.4 mm); load 2.6 +/- 0.2 lbs (11.6 +/- 0.9 N) compressed to 0.5 inch
4. Check valve seat and clean away any accumulation of metal particles.
5. Verify valve seat is concentric with larger diameter valve bore.
6. Clean plunger and spring in parts cleaning solvent and install.

⚠ Adjust the valve clearance only when engine is at ambient temperature.

Tools: feeler gauge, 7/16-inch open end wrench, 9/16-inch (14 mm) open end wrench, socket wrench

1. Remove ignition key to prevent accidental starting.
2. Remove all parts necessary to gain access to valve tappets.
3. Remove spark plugs to ease turning the engine over by hand.
4. Place a socket wrench on the flywheel capscrew and rotate crankshaft clockwise until left intake valve opens and closes; continue until TC mark on flywheel lines up with TC mark on gear cover (left pi
5. Verify left intake and exhaust valves are closed and there is no pressure on valve lifters.
6. Correct feeler gauge should pass freely between valve stem and tappet; a 0.002 inch (0.05 mm) thicker gauge should not.
7. To correct clearance, use a 7/16-inch open end wrench to turn adjusting screw; a 9/16-inch (14 mm) open end wrench is required to hold the tappet.
8. To adjust valves on the right hand cylinder, turn engine one complete revolution and again line up the marks.
9. Replace all parts removed and tighten all screws securely; torque manifold bolts to specified torque.

Tools: compression gauge

1. Run the engine until thoroughly warm.
2. Stop engine and remove spark plugs.
3. Remove air cleaner and place throttle and choke in the wide open position.
4. Insert the compression gauge in one spark plug hole.
5. Crank the engine and note the reading.
6. Refer to SPECIFICATIONS section for compression pressures.