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Cummins/Onan J120-S/967F Engine

The Cummins/Onan J120-S/967F is a 1.98 L 4-cylinder air cooled engine rated at 1800 rpm. It powers generator sets from Hol-Gar, including the GS-107-AC.

Also known as: ONAN, Division of Studebaker Corporation J120-S/967F

Part of the Cummins/Onan J120 engine family — 3 variants share parts & fitment.

1.983 L

Displacement

Cylinders

1800 rpm

Rated speed

On this pageSpecs Generator sets Maintenance FAQ Service intervals Torque & specs Fluids Procedures Wiring

Engine specifications

Displacement	1.983 L
Cylinders	4
Aspiration	Air Cooled
Rated speed	1800 rpm

Generator sets powered by this engine

Hol-Gar GS-107-AC

How to read your Cummins/Onan engine serial & model number →

Common problems

Tapping sound, clacking, light clicking
Cause: Metallic knock und
Pinging sound when engine is suddenly or heavily loaded
Cause: Engine misfiring, Excess oil in crankcase, Engine overheating, Defective intake valve stem cil seal, Worn intake valve, guide or valve stem, Faulty ignition, Oil leak from oil base or connections, Leaky crankshaft oil seals, Fuel mixture too rich, Choke not open, Clogged air intake, Worn crankshaft bearing, Low oil supply, Oil badly diluted, Valve clearance too great, Broken valve spring, Worn connecting rod bearings, Loose piston
Fix: See Engine Misfiring this chart, Drain to full level, See para. 10b., Replace (para. 16), Replace (para. 16), See Trouble-Shooting Chart 3, Inspect engine; check for clogged crank-case breather, Inspect oli seals, para 18b. (2) and 18e. (5). Check for clogged crankcase breather, Adjust carburetor, para Lle., Clean air cleaner, para lid, Replace bearing, Add oil, check for cause of low off, Replace oil, Remove carbon, Adjust breaker points. Retime spark if necessary
Load Switch-cireuit breaker trips.
Fix: Check and adjust load. Resat load switch If condition con- tinues shut down unit and check external and internal circuits.
Light pounding knock
Cause: Defective intake valve stem cil seal, Worn intake valve, guide or valve stem, Faulty ignition, Oil leak from oil base or connections, Leaky crankshaft oil seals
Fix: Inspect engine; check for clogged crank-case breather, Inspect oli seals, para 18b. (2) and 18e. (5). Check for clogged crankcase breather
Hollow clicking sound with engine cool and under load
Cause: Engine misfiring
Fix: See Engine Misfiring this chart
Generator does not build up and produce voltage.
Cause: Blackened collector rings. Intermittent sparking at brushes.
Fix: Sand or turn down collector rings. Replace short brushes. Adjust springs - replace weak brush springs.
Frequency cannot be maintained or engine keeps racing and/or slowing down, Frequency does not stabilize.
Cause: Excessive changing load. Governor cannot control engine speed: out of adjustment or defective.
Fix: Check load and adjust as re-quired. Adjust engine governor. Replace governor if adjustment cannot be made.
Excessive hum.
Cause: Uneven air gap. Foreign matter in air gap. Extreme vibration. Alignment of unit Dirt or alignment.
Fix: Check brushes. Measure with feeler gauge. Check roter for balance. Check bearing. Remove rotor and clean. Check brushes and spring set. Tighten mounting bolts to base and engine. Blow out all dirt in unit, use solvent on wound sections to remove Sticky dirt. Check flaw of ventilating air, Check for grounds.
Excess oil consumption, no increase in Smoke
Cause: Spark plug gap too wide
Fix: Adjust to .025
Excess oil consumption, light blue smoky exhaust
Cause: Defective high tension cables
Fix: Inspect and replace if necessary
Engine will not tarn over and fire.
Cause: Loose or defective wiring.
Fix: Tighten all loose connections ar replace.
Engine turns but will not start (Contd).
Cause: Engine flooded. Carburetor passages restricted by water, icé or corrosion. Fuel pump defective. Restricted air cleaner.
Fix: Push choke control all the way in and turn engine over, [f carburetor continues to flood, repair if, Clean fuel system. Replace or repair carburetor. Replace fuel pump. Remove air cleaner and clean filter.
Engine overheats
Cause: Wrong spark plug, Valve hot, Fuel stale or low octane, Lean fuel mixture, Engine hot, Defective ventilating system, Defective installation, Improper lubrication, Late ignition timing, Fuel mixture too lean
Fix: Install correct spark plug, Adjust valve clearance, Use good, fresh fuel, Clean and adjust carburetor, Check air circulation, Check engine and ducts, Inspect complete installation including ducts ete., Check oil system, Retime ignition, Adjust carburetor (para 11e). Clean Spark plugs and period of time on gasoline operation
Engine misses at light load
Cause: Incorrect ignition timing
Fix: See Trouble-Shooting Chart 3
Engine misses and backfires but fails to start
Cause: Incorrect timing
Fix: See Trouble-Shooting Chart 6 and para 18b.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan J120-S/967F.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Dry Foam Type Cleaner		—	200 hr
Fuel filter			—
Oil filter	Oil Filter Element		—	506 hr
Battery			—
Belt			—
Coolant		8.0 qts	—
Coolant filter	Cleaned or replaced		—
Oil	Lubrication Oil	8.0 qts	—	100 hr
Spark plug	Spark Plug		—	50 hr

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Service intervals

Item	Action	Hours	Notes
Cylinder head bolts and valve clearance	Adjust	50 hr	First service — Retighten head bolts and adjust valve clearance after first 50 hours on a new or overhauled engine
Breaker points	Check	100 hr
Crankcase breather	Clean	100 hr
Crankcase oil	Change	100 hr	Change when warm; more often under extreme dust or cold
Fuel sediment bowl	Drain	100 hr	Empty accumulated sediment and clean screen
Governor linkage	Lubricate	100 hr	Powdered graphite or light sewing machine oil
Spark plugs	Clean	100 hr	Clean and set gap
Air cleaner (foam type)	Clean	200 hr	Wash element; saturate with SAE 20W oil
Generator group	Inspect	250 hr	Inspect brushes, slip rings, commutator, mountings
Lubricating oil filter	Change	500 hr
Spark plugs	Replace	500 hr	Also clean combustion chamber and manifold; adjust governor and throttle linkage
Generator brushes	Check	750 hr	Check brush length; clean slip rings and commutator

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Service specifications

Type	Item	Value	Unit	Condition
Clearance	Cam tappet diameter	.7475/.7480	in
Clearance	Cam tappet hole diameter	.7505/.7515	in
Clearance	Camshaft bearing clearance limit	.0012/.0037	in
Clearance	Camshaft end play	.007/.038	in
Clearance	Connecting rod bushing clearance (pin in rod)	.0002/.0007	in
Clearance	Connecting rod journal diameter	2.0660/2.0605	in
Clearance	Connecting rod large bearing to crankshaft	.001/.003	in
Clearance	Crankshaft end play	.010/.015	in
Clearance	Crankshaft main bearing clearance	.0024/.0044	in
Clearance	Crankshaft main bearing journal diameter (STD)	2.2430/2.2435	in
Clearance	Cylinder diameter limits (honed)	3.2495/3.2505	in
Clearance	Piston clearance to cylinder wall	.002/.004	in	selective fit, 1/4 inch below oil ring at top of skirt
Clearance	Piston pin to piston	Thumb push fit		room temperature
Clearance	Piston to cylinder	.0008/.0028	in	1-1/4 inch from bottom of skirt
Clearance	Valve seat width	3/64 - 1/16	in	45 degree seat angle
Gap	Magneto breaker point gap (WICO)	.015	in
Gap	Piston ring end gap	.010/.020	in
Gap	Spark plug gap	.025	in
Other	Firing order	1-2-4-3
Other	Ignition timing spark advance	25	degrees BTC	up to 2700 rpm
Pressure	Cylinder compression	110	psi	new engine cranking at about 300 rpm; less than 10 psi difference between cylinders
Pressure	Fuel pump pressure	2-3	psi	gauge about 16 in above fuel pump
Pressure	Minimum oil pressure	20	psi	half throttle or governed speed
Pressure	Normal oil pressure	25 or higher	psi	engine at operating temperature
Pressure	Oil by-pass valve spring force	2.25 +/- 1/4	lbs	at 1-3/16 in
Pressure	Valve spring load, valve closed	45-49	lbs
Pressure	Valve spring load, valve open	83-93	lbs
Speed	Governed engine speed	1800	rpm
Temperature	Valve clearance check temperature	70	F	room temperature
Torque	Center main bearing bolt	87-102	lb-ft
Torque	Connecting rod bolt	27-29	lb-ft
Torque	Cylinder head bolt	28-30	lb-ft
Torque	Exhaust manifold nuts	13-15	lb-ft
Torque	Flywheel mounting screw	65-70	lb-ft
Torque	Fuel pump mounting screws	15-20	lb-ft
Torque	Gear cover	15-20	lb-ft
Torque	Intake manifold	13-15	lb-ft
Torque	Oil base mounting screws	45-50	lb-ft
Torque	Oil pump mounting screws	15-20	lb-ft
Torque	Rear bearing plate	40-45	lb-ft
Torque	Rocker arm nut	4-10	lb-ft	friction between threads only; locks nut in place
Torque	Rocker arm stud	18-20	lb-ft
Torque	Rocker box cover	8-10	lb-ft
Torque	Spark plug	25-30	lb-ft
Valve clearance	Exhaust valve clearance	0.015	in	engine at room temperature (approx 70 F)
Valve clearance	Intake valve clearance	0.012	in	engine at room temperature (approx 70 F)
Voltage	Voltage regulator range, 125% load	112-134	volts	120 volt base, +/-3%
Voltage	Voltage regulator range, full load	114-126	volts	120 volt base, +/-5%
Voltage	Voltage regulator range, no load	121-133	volts	120 volt base, +/-5%

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Recommended fluids

Fluid	Grade	Standard	Capacity	Notes
Engine oil	SAE 10W	MIL-L-2104		0 to 30 F
Engine oil	SAE 30	MIL-L-2104		30 to 90 F
Engine oil	SAE 50	MIL-L-2104	6.0 qts	Above 90 F; detergent oil classified DG or MS/DG
Engine oil	SAE 5W	MIL-L-10295		Below 0 F
Fuel	Regular grade gasoline		5.3 gals	Do not use highly leaded premium grades; never fill tank while engine is running
Grease	Powdered graphite (or light non-gumming/sewing machine oil)			Governor linkage ball joints and link-to-throttle

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Procedures

Adjusting governor speed and sensitivity adjustment

Tools: Tachometer, Screwdriver

Hold sensitivity stud with a screwdriver.
Turn speed adjusting nut clockwise to increase speed.
Use a tachometer to indicate engine speed.
Relocate spring in governor arm notches for coarse sensitivity.
Turn sensitivity stud counterclockwise to increase sensitivity.
Adjust for maximum sensitivity without hunting.

Adjusting the carburetor adjustment

⚠ Do not force needle valves against their seats

Warm up engine; verify ignition system and governor are adjusted.
With engine idling, turn idle adjustment out until speed drops slightly.
Turn needle in until speed returns to normal.
Apply a full load to the engine.
Override governor and check acceleration.
Adjust main needle outward 1/2 turn; repeat until no lag.
Set throttle stop screw for 1/32 inch clearance at no load.

Adjusting valve clearance adjustment

Tools: Feeler gauge

Check clearance with engine at room temperature (about 70 F).
Turn flywheel so cylinder is 10 degrees past TDC of compression stroke.
Adjust clearance with the self-locking nut.
Set intake clearance to 0.012 inch and exhaust to 0.015 inch.

Changing output voltage connection adjustment

⚠ Always check change plates are properly connected for intended use and are secure

Select the plate or plates for the desired output.
Secure plate(s) to the connection board as shown on the diagram.
Make load connections at the load terminals as indicated.
Store unused plates on the studs of the connection board cover.
Replace cover.

Timing the magneto to the engine adjustment

⚠ Do not disassemble magneto unless witness marks are made for correct reassembly

Tools: Timing light

Remove No. 1 spark plug; turn flywheel until pressure builds.
Continue turning until the 25 degree mark aligns with timing pointer.
Turn magneto until white mark on rotor is visible in window.
Mount magneto on studs with gasket; mesh gears.
Run engine and use timing light for exact timing.
Tighten magneto mounting securely.

Changing crankcase oil maintenance

⚠ Do not start engine if oil is too cold to flow

Run engine until oil is warm.
Drain oil from the oil base.
Reinstall drain plug.
Fill crankcase to FULL mark with proper grade (6 quarts).
Check for leaks.

Fitting new generator brushes maintenance

⚠ Do not use emery or conductive abrasive cloth

Tools: Grade OO flint paper

Slip grade OO flint paper between contact surface and brush, flint facing brush.
Move paper in one direction only; lift and return.
Repeat until proper brush surface is obtained.
Seat all brushes; new brushes may spark until seated.
Operate generator at light load to wear brushes in.

Replacing the oil filter maintenance

Tools: Oil filter wrench (box or strap type)

Place a drip pan below the filter.
Unscrew oil filter counterclockwise by hand or with a filter wrench.
Clean filter mounting area.
Oil the new gasket and install new filter hand tight.
Turn an additional 1/4 turn with a filter wrench.

Servicing the air cleaner maintenance

Remove the foam element.
Wash element thoroughly in gasoline or diesel fuel.
Saturate with SAE 20W motor oil.
Squeeze as dry as possible.
Reinstall element; replace only if damaged.

Applying load operation

Allow unit to warm thoroughly before connecting load.
Apply load in steps (1/4, 1/2, 3/4, full) at 30 minute intervals.
Place load switch (circuit breaker) in ON position.
Check voltage and adjust with voltage adjustment knob.
Check meters for proper readings; if overload, stop unit and adjust load.

Starting the engine (manual crank) operation

Tools: Hand crank

Prime carburetor with fuel pump priming lever about 15 strokes; return lever inward.
Set engine switch in START position.
Set hand choke as required by local temperature.
Engage hand crank with handle at 7 o'clock position.
Pull crank to 12 o'clock with sharp steady pull; remove when engine starts.
Place engine switch in RUN position.
Check oil pressure; it should be at least 20 psi.
Readjust choke as engine warms.

Stopping the engine operation

⚠ Stop button at engine magneto can also ground magneto to stop engine; return panel switch to STOP

Disconnect as much load as possible.
Place load switch (circuit breaker) in OFF position.
Allow engine to idle a short time.
Place engine switch in STOP position.
Place fuel valve in OFF position.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Voltage reconnection

Voltage	Phase	Connection	Notes
120	1	Delta, 2 wire	12.5 kVA, 104 A
120/208	3	Wye, 4 wire	18.75 kVA, 52 A
120/240	1	Delta-Delta, 3 wire	12.5 kVA, 52 A
240	3	Delta, 3 wire	18.75 kVA, 45 A
240/416	3	Wye, 4 wire	18.75 kVA, 26 A

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Parts cataloged across the Cummins/Onan J120 family

Part #	Description	Category
140B640	Element, Air Cleaner (foam type)	Air filter
140B641	Retainer, Air Cleaner Element	Air filter
19181	Voltage Regulator Complete with Rheostat and Knob	Avr
19181-1	Element, Regulator Plug-In	Avr
18383	Brush, Exciter	Brush
18773	Brush, AC	Brush
101K358	Gasket Kit, Rear Bearing Plate	Gasket
102B475	Gasket, Oil Base	Gasket
103C218	Gasket, Backplate	Gasket
110B1223	Gasket, Cylinder Head	Gasket
140C584	Gasket, Air Cleaner	Gasket
141A281	Gasket, Carburetor Flange	Gasket
149K875	Repair Kit, Fuel Pump (diaphragm assy, valves & gaskets)	Gasket
149P517	Gasket, Fuel Pump Bowl	Gasket
120B561	Line, Oil, Rocker Cover	Hose
19306	Hose, Auxiliary Fuel (10 ft)	Hose
19307	Fitting, strt - .500FNPT x M22 x 1.50	Hose
110B1413	Valve, Intake	Service part / kit
113A80	Ring, Piston, Top Compression (STD)	Service part / kit
114-181	Bearing, Connecting Rod, Upper or Lower Half	Service part / kit
120A539	Valve, Oil By-pass	Service part / kit
120A555	Spring, Oil By-pass	Service part / kit
143K241	Repair Kit, Carburetor	Service part / kit
162-227	Condenser (WICO X6916)	Service part / kit
162-246	Coil (WICO X5700B)	Service part / kit
162-247	Rotor (WICO Y5710)	Service part / kit
410A1221	Spring, Valve	Service part / kit
J10A860	Valve, Exhaust, Stellite	Service part / kit
120B547	Pump Assy, Oil	Pump
509-86	Seal, Oil - Crankshaft Rear	Seal
509-87	Seal, Oil - Gear Cover	Seal
509A90	Seal, Oil - Intake Valve	Seal
509P88	Seal, Oil, Governor Shaft	Seal
193P122	Gauge, Oil Pressure	Sensor
809A169	Switch, Low Oil Pressure Cut-off	Sensor
14P32	Suppressor, Spark Plug	Spark plug
187-4	Plug, Spark (Champion H-8 or Equal)	Spark plug

Parts cataloged across all variants of this engine family — verify the exact part fits your specific variant and serial number before ordering.

Common questions

What is the displacement of the Cummins/Onan J120-S/967F?

The Cummins/Onan J120-S/967F has a displacement of 1.983 liters with 4 cylinders.

Which generators use the Cummins/Onan J120-S/967F engine?

The Cummins/Onan J120-S/967F powers 1 generator set(s), including Hol-Gar GS-107-AC.

How often should you change the oil in the Cummins/Onan J120-S/967F?

The Cummins/Onan J120-S/967F oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan J120-S/967F engine?

A commonly reported issue on generators using the Cummins/Onan J120-S/967F is: Tapping sound, clacking, light clicking. Cause: Metallic knock und.