Cummins/Onan B48 Generator Set

⚠ 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.

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.

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.

Tools: Plastigauge, torque wrench

1. Make certain all parts are marked or identified so they are reinstalled in their original positions.
2. Place a piece of correct size Plastigauge in the bearing cap the full width of the crankshaft rod surface and about 1/4 inch (6.35 mm) off center.
3. Rotate the crank about 30 degrees from bottom dead center and reinstall the bearing cap; tighten bolts to specified torque; do not turn the crankshaft.
4. Remove bearing cap; the flattened Plastigauge will adhere to either the bearing cap or crankshaft.
5. Compare flattened Plastigauge with the graduations on the Plastigauge envelope to determine clearance.

⚠ Reversing positive and negative battery connections or allowing engine to run without being connected to the alternator will result in engine electrical system damage.

Tools: volt-ohmmeter such as Simpson 270, DC voltmeter

1. With engine running, check battery terminal voltage (regulator output) using a DC voltmeter; should be 13.6 to 14.7 VDC.
2. If voltage is greater than specified replace regulator-rectifier; if less, proceed to step 2.
3. Examine all wires for loose, corroded, broken connections, short circuits; check fuses; repair as needed.
4. Disconnect plug from regulator-rectifier and test AC voltage at plug with engine running near 3600 rpm.
5. If AC voltage reads more or less than specified, proceed to step 4; if AC voltage is as specified but DC voltage low, replace regulator-rectifier.
6. Use Rxl scale on ohmmeter to detect opens in stator (unit not running); refer to Table 1 for resistance specifications.
7. If resistance is not as specified, replace stator; if windings are grounded the stator must be replaced.

⚠ Ignition of fuel can result in severe personal injury or death. Thoroughly clean up any spilled fuel.

Tools: vacuum gauge, pressure gauge, fuel hose with clamps

1. Operate engine at an idle for five minutes to ensure carburetor is full of fuel.
2. Shut engine off and remove fuel inlet line from fuel pump.
3. Connect a vacuum gauge to fuel pump inlet using a piece of fuel hose with clamps.
4. Start engine and allow to idle for at least five seconds; record vacuum gauge reading.
5. Move throttle control to high idle position, wait at least five seconds, record vacuum gauge reading.
6. Shut engine off and remove vacuum gauge hose; reconnect fuel inlet line.
7. Remove fuel outlet line from fuel pump.
8. Connect a pressure gauge to fuel pump outlet using a piece of fuel hose with clamps.
9. Start engine and idle at least five seconds; record pressure gauge reading holding gauge level with pump outlet.
10. Move throttle to high idle and run at least five seconds; record pressure reading.
11. Shut engine off and remove pressure gauge hose; reconnect fuel outlet line.
12. Repair or replace the fuel pump if test readings are not within TABLE 1 values.

Tools: Simpson 260 VOM or equivalent

1. Make sure ignition power is off and coil is at room temperature of 70 deg F (21 deg C).
2. Use a Simpson 260 VOM or equivalent.
3. Place black lead on negative (-) terminal of coil and red lead to positive (+) coil terminal; primary resistance should read between 3.87-4.73 ohms.
4. Change resistance setting on ohmmeter; place ohmmeter leads inside of spark plug cable holes; secondary resistance should read 12,600-15,400 ohms.
5. If any of the above conditions are not met, replace coil.

Tools: ohmmeter or continuity test lamp

1. Adjust breaker points.
2. Remove blower housing to expose timing marks on top of gearcase cover and flywheel.
3. Rotate flywheel clockwise until timing mark is aligned with the proper number of timing degrees stamped on top of gearcase cover.
4. Connect an ohmmeter or continuity test lamp across the ignition breaker points; one prod to coil lead terminal, other to a good ground.
5. Turn crankshaft counterclockwise until points close, then slowly turn clockwise.
6. Lamp should go out or continuity lost just as points break.
7. If timing is early (advanced) point gap is too large; if late (retarded) point gap is too small; adjust accordingly.

Tools: feeler gauge

1. Remove spark plugs.
2. Pull starter pinion gear outward on its shaft until pinion gear teeth mesh completely with flywheel ring gear teeth.
3. Measure free travel (lash) between pinion gear teeth and ring gear teeth.
4. Adjust starter motor as necessary to obtain 0.020 +/- 0.010 inch lash.