Cummins/Onan NHC-NHCV Engine

⚠ Forcing mixture adjustment screws tight will damage needle and seat. Turn in only until light tension can be felt.

Tools: screwdriver, tachometer

1. Turn mixture screws in until lightly seated, then back out turns specified in Table 1.
2. Start engine and warm thoroughly at least 10 minutes.
3. Move speed control to slow position.
4. Pull governor back so throttle stop screw is against stop.
5. Adjust throttle stop screw to obtain 1200 rpm, or 1100 rpm if low speed screw exists.
6. Set idle adjustment screw about 1/8 turn outward from midpoint of maximum speed range.
7. Re-adjust throttle stop screw to specified rpm and release governor arm.
8. Check main mixture by rapidly accelerating; adjust if hesitation.

1. Ensure ignition and governor are adjusted.
2. Start and warm engine.
3. If engine will not start, close both needle valves gently, then open each 1 to 1-1/2 turns, idle first then main.
4. With no load, turn idle adjustment out until speed drops slightly below normal; turn in until speed returns to normal.
5. Apply full load, turn main adjustment in until speed drops slightly below normal; turn out until speed returns to normal.

1. Perform steps 1 and 2 of Adjustment Under Load.
2. Pull governor mechanism to slow unit to about 400-500 rpm.
3. Set idle adjustment screw for even operation.
4. Release governor to accelerate; adjust main needle outward about 1/4 turn if needed.
5. Set throttle stop screw at desired idle speed.

⚠ Attempting adjustments with float assembly installed may deform inlet needle and seat.

Tools: screwdriver, measuring gauge

1. Disconnect throttle control, choke leads, breather hose, air cleaner inlet hose, and fuel line from carburetor.
2. Remove four intake manifold bolts and remove carburetor and manifold as one unit.
3. Remove carburetor from intake manifold.
4. Separate upper body from fuel bowl section.
5. Measure float level.
6. If incorrect, remove float assembly and bend tang slightly as specified in Figure 13.
7. Reassemble and install.

1. Remove fuel line, air cleaner hose, governor linkage, and choke wires.
2. Remove two carburetor mounting nuts and carburetor.
3. Unscrew float bowl nut and remove main fuel adjustment assembly.
4. Invert carburetor and check float surface nearest gasket is 1/8 ± 1/16 inch from gasket.
5. Bend float tab as required.
6. Reassemble, install, and test.

Tools: Tachometer

1. Run engine about 15 minutes to normal operating temperature.
2. Adjust carburetor main jet for best full load mixture.
3. Adjust carburetor idle needle with no load.
4. Adjust length of governor linkage.
5. Check governor linkage and throttle shaft for binding or excessive looseness.
6. Adjust governor spring tension for nominal no load speed.
7. Check rpm drop between no load and full load and adjust sensitivity.
8. Recheck speed adjustment.
9. Set carburetor throttle stop screw.

Tools: Continuity test lamp

1. Connect continuity test lamp across breaker points.
2. Touch one prod to breaker box terminal with coil lead and other to ground.
3. Turn crankshaft counterclockwise until points close.
4. Slowly turn crankshaft clockwise.
5. Lamp should go out just as points break, indicating ignition at 25° BTC.

Tools: Timing light

1. Connect timing light per manufacturer instructions.
2. Start engine and check timing at idle.
3. Pointer on flywheel should line up with 25° mark on cover.
4. If marks do not line up, readjust point gap.
5. To advance timing, slightly open breaker point gap.
6. To retard timing, slightly close breaker point gap.
7. Recheck timing and point gap.

Tools: feeler gauge, torque wrench

1. Ensure starter mounting surfaces are free of dirt and oil.
2. Install starter and tighten mounting bolts just enough to hold in place.
3. Remove spark plugs to allow free flywheel movement.
4. Manually pull starter pinion outward to fully mesh with flywheel ring gear.
5. Measure free travel (lash) between pinion and ring gear teeth.
6. Proper lash is 0.020 ± 0.010 in. (0.51 ± 0.25 mm).
7. Loosen and adjust starter as necessary.
8. Torque mounting bolts and reinstall spark plugs.

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

Tools: Feeler gauge

1. Remove parts to access valve tappets.
2. Remove spark plugs.
3. Turn engine by hand until left intake valve opens and closes and TC mark aligns.
4. Ensure both valves closed and left piston at top of compression stroke.
5. Check clearance with gauge between valve stem and tappet.
6. Adjust with self-locking screw to correct clearance.
7. For right cylinder, turn engine one revolution and repeat.
8. Replace parts and torque manifold bolts.

1. Shut down engine at least one hour; adjust at ambient temperature.
2. Loosen screw securing choke body assembly.
3. Rotate choke body clockwise to richen or counterclockwise to lean.
4. For ambient above 85°F, choke should be fully opened.
5. For ambient below 25°F, choke should be opened 1/4 inch with solenoid not engaged.
6. Tighten choke body screw.

Tools: feeler gauge, 7/16-inch open end wrench, 9/16-inch open end wrench

1. Remove ignition key and spark plugs.
2. Remove parts to access valve tappets.
3. Rotate crankshaft clockwise until left intake valve opens and closes.
4. Continue turning until TC mark on flywheel aligns with TC mark on gear cover.
5. Verify left intake and exhaust valves are closed and no pressure on lifters.
6. Check clearance with correct feeler gauge; 0.002 inch thicker gauge should not pass.
7. Adjust with 7/16-inch open end wrench; hold tappet with 9/16-inch wrench.
8. For right cylinder, turn engine one revolution and repeat.