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Cummins/Onan J block (Series JB/JC) Engine

The Cummins/Onan J block (Series JB/JC) is a 1.2 L 2-cylinder air cooled engine rated at 1500 rpm. It powers generator sets from Cummins/Onan, including the YD series.

Also known as: Onan J block (Series JB/JC)

1.2 L

Displacement

Cylinders

1500 rpm

Rated speed

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

Engine specifications

Displacement	1.2 L
Cylinders	2
Aspiration	air cooled
Rated speed	1500 rpm

Generator sets powered by this engine

Cummins/Onan YD series

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

Common problems

The engine will not crank.
Cause: Possible fault with control or starting system.
Fix: Check for the following conditions: a. Fuse F1 has blown. Replace the fuse with one of the same type and amp rating. b. Fault circuit breaker has tripped. Service the fault, and reset the circuit breaker. c. Poor battery cable connections. Clean the battery
The engine cranks, but does not start or starts and then shuts down.
Fix: Check for empty fuel tank, fuel leaks, or plugged fuel lines and correct as required. Check for dirty fuel filter and replace if necessary (see Maintenance section). Check for dirty or plugged air filter and replace if necessary (see Maintenance section). Check ignition system (see Maintenance section). Contact an Onan distributor for assistance if none of the above.
Partial loss of residual in rotor.
Fix: With plant operating, jumper from E2 to heat sink of field rectifier Z until voltage begins to build up. Then remove.
Pair of field rectifiers open (either W and Z or X and Y).
Fix: Test rectifiers and replace if defective.
Over-Voltage or Fluctuating Voltage
Cause: The Magneciter is probably defective.
Fix: If the engine is operating at the correct speed, see Magneciter Troubleshooting Chart.
Output voltage slow to build up.
Cause: Either field rectifier X or Y shorted.
Fix: Test rectifiers and replace if defective.
Output voltage slow to build up and higher than rated voltage after build up.
Cause: Open circuit in one or more control rectifier.
Fix: Test rectifiers and replace if defective. Check soldered connections to rectifiers.
Output voltage slow to build up and 10 to 20 percent above rated voltage after build up.
Cause: Open circuit in one field rectifier.
Fix: Test rectifiers and replace if defective.
Output voltage builds up normally with slightly less than rated voltage at no load and 20 percent above rated voltage at full load.
Cause: Compound winding S1-S2 in- stalled backward or has open circuit through one of compound windings through reactors A and B and test for continuity.
Output voltage builds up normally but less than rated voltage at no load and low voltage at full load.
Cause: Shorted winding in control reactor.
Fix: Test control reactor and replace if defective.
Output voltage builds up normally but is 125 to 150 percent above rated voltage after build up.
Cause: Shorted turn in gate winding Gi-G2 of reactor A or B.
Fix: Test reactors A and B for shorted turns and replace if defective.
Output voltage builds up normally but 25 percent above rated voltage after build up: No regulation possible.
Cause: Open circuit in control rectifier bridge.
Output voltage builds up normally but 150 to 200 percent above rated voltage after build up. No regulation possible.
Cause: Shorted turn in control winding C1-C2 or reactor A or B.
Fix: Check circuit connections of both reactors A and B.
Output voltage builds up normally but 150 to 200 percent above rated voltage after build up.
Cause: Control winding C1-C2 or reactor A or B polarized incorrectly.
Fix: Check circuit connections of both reactors A and B.
Open in circuit breaker.
Fix: Stop plant and check breaker continuity.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan J block (Series JB/JC).

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Replace air filter at each oil change.		—	50 hr
Fuel filter			—	200 hr
Oil filter	Replace oil filter at each oil change.		—	100 hr
Battery	Check battery electrolyte level and charge voltage every 100 hours or as needed.		—	100 hr
Belt			—	1000 hr
Coolant			—	200 hr
Oil	Change oil and filter every 100 hours or as needed.	3.5 quarts	—	100 hr
Spark plug	Replace spark plugs at each oil change.		—	200 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	Months	Notes
Exhaust system	Inspect	8 hr		Daily
Generator set	Inspect	8 hr		Daily
Air cleaner	Check	50 hr
Battery	Check	50 hr		Check condition of the battery
Crankcase oil	Change	100 hr		First service — Change crankcase oil after the first 50 hours
Cylinder head bolts	Adjust	50 hr		First service — After the first 50 hours of operation, retighten the cylinder head bolts
Valve clearance	Check	50 hr		First service — After the first 50 hours of operation, recheck valve clearance
Air cleaner (dry paper cartridge)	Clean	100 hr		Do not wash; replace at least every 500 hours
Battery water level and charge condition	Check	100 hr
Fuel filter	Clean	100 hr		Fuel filter sediment bowl
Governor linkage	Clean	100 hr		Clean and adjust (governor rod ball joints)
Air cleaner (foam element)	Clean	200 hr		Replace only if damaged
Crankcase breather	Clean	200 hr
Crankcase breather	Replace	200 hr		Series JB
Generator	Clean	1000 hr		Clean the generator
Ignition breaker points	Adjust	200 hr		Series JC; clean and adjust
Oil filter	Replace	200 hr		Normally every 200 hours; shorten if oil very dirty
Spark plug	Check	200 hr		Check the spark plug gap
Build-up relay (02SX exciters)	Clean	500 hr		02SX exciters require build-up relay cleaning
PCV valve	Clean	500 hr		Series JC
PCV valve (JC begin Spec T)	Clean	500 hr		Wash in diesel fuel or kerosene
Starter disconnect breaker	Replace	500 hr		Service the starter disconnect breaker
Voltage build-up relay (02SX exciters)	Clean	500 hr		J-series generator sets using 02SX exciters
Ball bearing	Inspect	1000 hr		Inspect every 1000 hours with the plant running
Brushes	Inspect	1000 hr		Revolving field generator
Collector rings	Inspect	1000 hr
Cooling fins (cylinder block and heads)	Clean	1000 hr		More often under dirty conditions
Exciter	Inspect	1000 hr		Periodic inspection of the exciter
Exciter, ball-bearing, collector rings, brushes	Inspect	1000 hr		at least every 1000 hours
Generator ballbearing	Inspect	1000 hr		with unit running
Oil pan	Clean	1000 hr		Remove and clean the oil pan
Rocker box oil line holes	Clean	1000 hr		Flush line with solvent, clean holes with fine wire
Valves	Adjust	1000 hr		Grind the valves if necessary - contact an authorized Onan dealer
Generating plant	Inspect	5000 hr		Complete reconditioning
Generator ball bearing (prime power)	Replace	10000 hr	24 mo	If used as prime power, replace bearing every 10,000 hours or two years
Generator ballbearing	Replace	10000 hr	24 mo	or every 5 years for standby power
Cylinder head bolts	Check			First service — Tighten head bolts after first 50 hours on new or overhauled engine
Generator ball bearing (standby power)	Replace		60 mo	If the plant is used for standby power, replace bearing every five years
Valve clearance	Adjust			First service — Adjust after first 50 hours on new or overhauled engine

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	Armature runout	less than .002	in.
Clearance	Brush wear limit	5/16	inch	replace when worn to this
Clearance	Camshaft end play	.007 - .039	in
Clearance	Camshaft to bearing clearance	.0012 to .0037	in.
Clearance	Collector rings T.I.R.	.002	inch
Clearance	Collector rings Total Indicated Reading (T.I.R.)	.002	in.
Clearance	Connecting rod bearing to crankshaft journal clearance	.001 to .003	in.
Clearance	Connecting rod bearing-to-crankshaft clearance	.001 - .003	in
Clearance	Connecting rod upper bushing to piston pin clearance	.0002 to .0007	in.
Clearance	Crankshaft end play	.010 - .015	in
Clearance	Crankshaft end play	.010 to .015	in.
Clearance	Crosshead to valve spring retainer	.025	inch	minimum
Clearance	Cylinder bore	3.250	in
Clearance	Distributor shaft side play	less than .005	in	5 lb pull
Clearance	Fuel pump coupling buffer end clearance	1/16	inch	minimum
Clearance	Fuel pump drive coupling alignment	.010	inch	at 4 equidistant points on pump spider
Clearance	Intake valve guide clearance	.001 - .003	in
Clearance	Main bearing clearance	.0014 - .0052	in
Clearance	Piston clearance (90 deg from piston pin axis, 3/8 in. below oil control ring)	.0012 - .0032	in.
Clearance	Piston ring gap	.010 - .020	in
Clearance	Starter armature end play	.005 - .030	in
Clearance	Starter pinion clearance	.09 +/- .02	in	target .07
Gap	Belt deflection	equal to thickness of belt for each foot of free span		approx 13 pounds index finger force
Gap	Centrifugal switch contact gap	.020	in.
Gap	Centrifugal switch point gap	0.040	inches
Gap	Distributor breaker point gap	.018 - .022	in	maximum gap
Gap	Ignition breaker point gap	.020	in	JB
Gap	Ignition breaker point gap	0.018-0.022	in (0.46-0.58 mm)
Gap	Ignition breaker point gap	0.019-0.021	in (0.48-0.53 mm)
Gap	Magneto pole shoe gap	.025	in
Gap	Piston ring end gap	.010 to .020	in.
Gap	Spark plug gap	.025	in	gasoline operation
Gap	Spark plug gap	0.025	in (0.64 mm)
Gap	Spark plug gap	0.035	in (0.89 mm)
Gap	Start-disconnect breaker point gap	.020	in
Gap	Starter disconnect breaker point gap	0.020	in (0.51 mm)
Other	Battery specific gravity when 75 percent charged	1.290 to 1.225		75 percent charged
Other	Center pin extension from camshaft gear	25/32	in.
Other	Cranking limiter limits cranking time	45 to 90	seconds	depending on ambient temperature
Other	Distributor rotation	Clockwise
Other	Firing order	1-2-4-3
Other	Governor stop pin distance from mounting face of cover	25/32	in.
Other	Ignition timing (gas and combination gas/gasoline)	35	degrees BTC at 1800 RPM	at 1800 RPM
Other	Ignition timing (gasoline)	25	degrees BTC at 1800 RPM	at 1800 RPM
Other	New cylinder dimensions	3.2495 to 3.2505	in.
Other	Oversize valve seat availability	.002, .005, .010 and .025	in.
Other	Piston and ring set oversize availability	.010, .020 and .030	in.
Other	Undersize bearings/connecting rods availability	.010, .020 and .030	in.
Other	Valve seat refacing angle and width	45 deg angle and 3/64 to 1/16	in.
Other	Voltage adjust rheostat adjustment	plus or minus 5	percent
Other	Voltage regulation	±3	percent	steady speed
Other	Voltage stabilization time	2	seconds	after a change in load
Pressure	Air inlet restriction at cleaner (maximum)	1.5	inches of mercury	at air cleaner outlet pipe
Pressure	Air inlet restriction at cleaner (maximum)	20	inches of water	at air cleaner outlet pipe
Pressure	Air inlet restriction at engine (maximum)	1.8	inches of mercury	rated speed, full load
Pressure	Air inlet restriction at engine (maximum)	25	inches of water	rated speed, full load
Pressure	Compression (standard new engine)	about 110	psi	cranking at about 300 rpm
Pressure	Compression uniformity	less than 10	psi	difference between highest and lowest cylinder at same cranking rpm
Pressure	Engine oil pressure	25 or higher	psi	at operating temperature
Pressure	Exhaust back pressure - maximum (inches of mercury)	3	inches of mercury	units of full load measurement

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	Ambient	Capacity	Notes
Coolant	Clean water, slightly alkaline	pH value range of 8.5 to 10.5		100.0	Keep chromate concentration between 100 and 150 grains per gallon; water should be free of chlorides, sulphates and acids; summer treatment one ounce chromate compound per two or three gallons of water
Engine oil	SAE 30	API MS, MS/DG, or MS/DM heavy-duty detergent		3.0 qt	For ambient 30F and above; JB capacity 3 qt plus 3/4 qt for filter; JC capacity 6 qt
Engine oil	SAE 50				Squirt a small amount into the cylinder to check for piston blow-by during the compression test
Engine oil	SAE 5W	Mil-L-10295	below 0°C		Use where temperatures are not above 0°F (value copied as printed in °F); may need one grade heavier in heavily loaded applications
Engine oil	SAE 5W-20	API MS, MS/DG, or MS/DM heavy-duty detergent			For ambient 30F and below
Engine oil	SAE viscosity grades per ambient temperature chart (10W-30, 10W-40, 20W-40, 20W-50, 30 shown on chart)	API service classification SF or SF/CC; must have passed MS Sequence Tests (or ASTM G-IV Sequence Tests); do not use Service DS oil		3.5 quarts	Includes 0.5 quarts (0.47 l) for the oil filter. Viscosity chart temperature range shown as -20 to 100 F.
Engine oil	SAE-30W	Mil-L-2104-A and/or British Defense Spec. DEF-2101B; Supplement I; Mil-L-2104-B; Series 3 (Mil-L-45199)			Recommended for normal conditions and where sulphur content of diesel fuel is 0.5% to 1% by weight; SAE 20W Series 3 premium oils not required except unusual conditions
Engine oil	SAE 10W		-10°C to 30°C		Temperatures consistently between -10°F and 30°F (units copied as printed)
Engine oil	SAE 20		20°C to 90°C		Temperatures consistently between 20°F and 90°F (units copied as printed)
Fuel	Gasoline/propane/natural gas				High compression pistons are necessary for full rated output on natural gas.
Fuel	Neutral distillate petroleum oil	Viscosity 2.4 to 5.0 centistokes (34 to 42 Saybolt Universal) @ 100°F; Gravity 30 to 42 degrees A.P.I. at 60°F; Cetane Number 40 minimum; Sulphur not to exceed 1% by weight			Pour point 10°F below lowest expected temperature; bottom sediment and water not to exceed .05% by weight; free from suspended matter
Fuel	Regular grade gasoline				Avoid premium high-lead fuels; change stored fuel every season
Grease	Multi-purpose industrial grease	ASTM Penetration, Normal Worked 285 to 320; Dropping Point 325°F; Water 1% maximum			Do not mix grades or brands; excessive lubrication is as harmful as inadequate lubrication

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

Procedures

Adjust Injectors and Valves adjustment

⚠ Injectors must always be adjusted before valves.

⚠ Be careful not to bruise injector tip.

Tools: Inch-pound torque wrench (Snap-On Model TQ12B or equivalent), Screwdriver adapter, Feeler gauge, Wire gauge

If used, pull compression release lever back and block open.
Bar engine in direction of rotation until No. 1 VS mark appears.
Adjust injector plunger, then crossheads and valves of first cylinder.
Turn crankshaft to next VS mark per firing order; adjust that cylinder.
Continue until all injectors and valves are adjusted.
Injector plungers adjusted with inch-pound torque wrench to torque setting.
Warm engine to 140°F oil temperature and reset injectors to warm setting.
Always make final valve adjustment after injectors and with engine at operating temperature.

Adjust air shutter (Vernatherm element) adjustment

⚠ Air housing door must be on when operating engine

Tools: Screwdriver

Loosen power element mounting screws in slotted holes
Slide assembly until plunger rests on roll pin with shutter closed
Shutter must be closed at ambient temperature
Test element by heating: plunger moves out at about 120F
Total plunger movement should be at least 13/64 in

Adjust gasoline carburetor adjustment

⚠ Do not force needle valves against seats

Tools: Screwdriver

Allow engine to warm up
At no load turn idle adjustment out until speed drops slightly
Turn needle in until speed returns to normal
Apply full load, turn main adjustment in until speed drops
Turn needle out until speed returns to normal

Adjust governor speed and sensitivity adjustment

⚠ Oversensitive adjustment may cause hunting

Tools: Reed frequency meter, Wrench

Hold governor spring stud, turn nut to set speed
Use a reed type frequency meter on generator AC output
Prior Spec R: turn spring stud counterclockwise for more sensitivity
Adjust for maximum sensitivity without hunting
Begin Spec R: turn speed nut clockwise to increase speed
Begin Spec R: turn ratchet nut counterclockwise to increase spring tension

Adjust valve clearance (four-cylinder) adjustment

Tools: Feeler gauge

Adjust valves in firing order 1-2-4-3
Set timing and clearance for cylinder 1
Turn flywheel 180 degrees for cyl 2, between 10 and 45 deg past BC
Adjust cylinder 2 clearance
Turn 180 degrees for cyl 4, between 10 and 45 deg past TC, adjust
Turn 180 degrees for cyl 3, between 10 and 45 deg past BC, adjust

Adjust valve clearance (two-cylinder) adjustment

⚠ Set clearances at room temperature, at least two hours after shutdown

Tools: Feeler gauge, Torque wrench

Turn flywheel to compression stroke after intake valve closes
Align TC mark with timing pointer, then turn clockwise 10 to 45 degrees
Check cylinder head bolt torque
Check rocker-arm-to-valve clearance with feeler gauge
Adjust with locknut to Table 3 clearance

Adjust valve clearance - Four-Cylinder J-Series engines adjustment

⚠ Early model four-cylinder engines do not have a BC mark on the flywheel

Tools: feeler gauge

Adjust valve clearance in firing order (1-2-4-3) sequence
Set timing for number 1 cylinder and adjust the valve clearance
Turn flywheel clockwise 180 degrees for number 2 cylinder (between 10 and 45 degrees past BC mark)
Adjust valve clearance for number 2 cylinder
Turn flywheel clockwise 180 degrees for number 4 cylinder (between 10 and 45 degrees past TC mark)
Adjust valve clearance for number 4 cylinder
Turn flywheel clockwise 180 degrees for number 3 cylinder (between 10 and 45 degrees past BC mark)
Adjust valve clearance for number 3 cylinder

Adjust valve clearance - Two-Cylinder J-Series engines adjustment

Tools: feeler gauge, torque wrench

Turn flywheel until cylinder to be adjusted is on its compression stroke
Turn flywheel until TC mark lines up with timing pointer, then turn clockwise an additional 10 to 45 degrees
Check cylinder head-bolt torque (see Fig. 60)
Using a feeler gauge, check clearance between rocker arm and valve
Increase or reduce clearance until proper gap is established, adjusting with the locknut
Refer to Table 3 for correct valve clearance setting

Battery Charge Rate Adjustment adjustment

⚠ Avoid overcharging.

Set slide tap on adjustable resistor R21 in generator air outlet to give about 2 amperes charging rate.
If charge winding AC output is below 19 volts on 12 volt battery charge models, 38 volts on 24 volt models, 50 volts on 32 volt models, test charging circuit for opens or grounds.
If a separate automatic demand control is used, adjust charge rate for maximum 4.5 amperes.

Check and Adjust Belt Tension adjustment

⚠ Never roll or tighten a belt over the pulley and never pry it on with a tool such as a screwdriver.

⚠ When two or more belts are used, they must be replaced as a set.

⚠ Do not tighten belt beyond figures given to eliminate belt squeak.

Shorten distance between pulley centers so belt can be installed without force.
Tighten belts so pressure of the index finger will depress belt to specified deflection.
All new belts will loosen after running an hour or more and must be retensioned.
If belt can be pushed down 1/8 inch more than recommended, retension.
To clean a belt, wipe with approved belt lubricant or hydraulic brake fluid.

Time ignition, plant running adjustment

⚠ With access door open engine overheats rapidly; run a few minutes without load only

Tools: Automotive timing light

Install automotive timing light on a spark plug lead
Run engine at rated speed and check timing
Loosen breaker plate mounting screws to correct timing
Rotate clockwise to advance, counterclockwise to retard
Tighten plate and recheck timing point

Time ignition, plant stopped (JB battery ignition) adjustment

Tools: Continuity test lamp, Battery

Disconnect ignition points lead, install continuity lamp and battery
Rotate flywheel clockwise until TC mark nears timing pointer
Rotate slowly until light goes out at 5 deg ATC
If incorrect, align 5 deg ATC mark and loosen breaker plate
Rotate plate until light goes out, tighten and check

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	Leads	Notes
110	1	518	V1	50 hertz
110/190	3	53C	V1	50 hertz
110/190	3	V1		50 Hz; decoded from Figure 3-4 (OCR degraded)
110/220	1	518	V1	50 hertz
110/220	1	53C	V1	50 hertz
110/220	1	V1		50 Hz; decoded from Figure 3-4 (OCR degraded)
110/220	3	518	V1	50 hertz
115	1	518	V2	50 hertz
115/200	3	53C	V2	50 hertz
115/230	1	518	V2	50 hertz
115/230	1	53C	V2	50 hertz
115/230	1	V2		50 Hz; decoded from Figure 3-4 (OCR degraded)
115/230	3	518	V2	50 hertz
118/200	3	V4		50 Hz; decoded from Figure 3-4 (OCR degraded)
120	1	3C	V1	60 hertz
120	1	518	V3	50 hertz
120, 240	1		2 wire	Revolving armature; Specs A-S
120, 240, 120/240	1	Reconnectible for 120, 240 or 120-240 Volts	4 wire	Revolving armature; Specs A-S
120, 240, 120/240	1	Revolving Field (3R) Reconnectible for 120, 240 or 120-240 Volt	3 wire	T2 must be grounded if field boost circuit is to work; any other ground may damage the exciter (negative ground only)
120/208	3		4 wire	Revolving armature; Specs A-S
120/208	3	3C	V1	60 hertz
120/208	3	53C	V3	50 hertz
120/208	3	V1		60 Hz; decoded from Figure 3-4 (OCR degraded)
120/208, 277/480, 240/416	3	Revolving Field, Wye Wound, 4-wire, 3-Phase (-4R 120-208, -4XR 277-480, -57XR 240-416)	4 wire	T1 must be grounded if the field boost circuit is to work (negative ground only)
120/240	1		3 wire	Revolving armature; Specs A-S
120/240	1	3C	V1	60 hertz. Connect X1 to VR21-5 for 50 hertz and X1 to VR21-6 for 60 hertz generators. Jumper W10 on VR21 must connect VR21-V4 to V1, V2, or V3 as listed.
120/240	1	518	V3	50 hertz
120/240	1	53C	V3	50 hertz
120/240	1	V1		60 Hz; decoded from Figure 3-4 (OCR degraded)
120/240	1	V3		50 Hz; decoded from Figure 3-4 (OCR degraded)

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

Common questions

What is the displacement of the Cummins/Onan J block (Series JB/JC)?

The Cummins/Onan J block (Series JB/JC) has a displacement of 1.2 liters with 2 cylinders.

Which generators use the Cummins/Onan J block (Series JB/JC) engine?

The Cummins/Onan J block (Series JB/JC) powers 1 generator set(s), including Cummins/Onan YD series.

How often should you change the oil in the Cummins/Onan J block (Series JB/JC)?

The Cummins/Onan J block (Series JB/JC) oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan J block (Series JB/JC) engine?

A commonly reported issue on generators using the Cummins/Onan J block (Series JB/JC) is: The engine will not crank.. Cause: Possible fault with control or starting system.. Fix: Check for the following conditions: a. Fuse F1 has blown. Replace the fuse with one of the same type and amp rating. b. Fault circuit breaker has tripped. Service the fault, and reset the circuit breaker. c. Poor battery cable connections. Clean the battery.