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Cummins/Onan BF Engine

The Cummins/Onan BF is a 0.66 L 2-cylinder engine rated at 3600 rpm, producing up to 11.9 kW. It powers generator sets from Cummins/Onan, including the 5.0BF - 34 3E and S.OSF - 34 IE. It holds 2 L (0.5 gal) of oil.

Also known as: Onan BF

0.66 L

Displacement

Cylinders

3600 rpm

Rated speed

11 kW

Gross power

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

Engine specifications

Oil capacity	2.0 L
Displacement	0.66 L
Cylinders	2
Rated speed	3600 rpm
Gross power	11 kW

Generator sets powered by this engine

Cummins/Onan 5.0BF - 34 3ECummins/Onan S.OSF - 34 IE

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

Troubleshooting

Fault codes

Code	Meaning	Cause	Remedy
F1	Fails to crank — blown fuse (F1)	Blown fuse (F1)	Replace fuse (F1) on control.
K2	Cranks but won't start — faulty crank ignition relay (K2)	Faulty crank ignition relay (K2)	Check voltage from relay terminal 87 to ground while cranking unit. Battery voltage should appear at this terminal. If not, check for voltage at relay terminals 30 and 85. If battery voltage is present at terminals 30 and 85 but not at 87, place a ground lead on terminal 86 and recheck. Replace relay if voltage is not present with ground lead in place. If no voltage is present at terminals 30 and 85 while cranking, check wiring between start solenoid (K1) and crank ignition relay (K2).
R1	Unit starts, but stops immediately after releasing start switch S1 — resistor R1 may be open	Resistor R1 may be open	Check voltage on both sides of R1. With set running, voltage should be 24-32 volts DC.
K3	Unit starts, but stops immediately after releasing start switch S1 — run ignition relay K3	Run ignition relay K3	Check voltage on both sides of K3. Should be 12 volts.
S3	Unit starts, but stops immediately after releasing start switch S1 — S3 low oil-pressure switch may be defective	S3 low oil-pressure switch may be defective	Check S3. Switch should close with set running at 10 lbs. minimum oil pressure.
F1	Meter or generator lamp inoperative — blown fuse (F1)	Blown fuse (F1)	Replace F1 fuse on control.

Symptoms & remedies

Symptom	Cause	Remedy
Fails to crank	Bad battery connection	Clean and tighten all battery and cable connections.
Fails to crank	Low battery	Check specific gravity. Recharge or replace if necessary.
Fails to crank	Faulty start solenoid (K1)	Check for battery voltage at K1 "I" terminal. Push start switch. Check start solenoid "S1" terminal voltage to ground. When battery voltage at start solenoid "B-" terminal is present, battery voltage should also appear at "S1" terminal. If not, check K3.
Fails to crank	Faulty start switch	Jumper switch (#3 terminal) to ground. If solenoid energizes, replace switch.
Cranks slowly	Bad battery connection	See 1 above (FAILS TO CRANK): Clean and tighten all battery and cable connections.
Cranks slowly	Low battery	See 2 above (FAILS TO CRANK): Check specific gravity. Recharge or replace if necessary.
Cranks but won't start	Faulty fuel solenoid or fuel pump	Fuel solenoid must open during cranking and running. Check by removing steel line from carburetor and crank engine. If fuel solenoid is open, fuel will pulsate out of this line. If it does not, the fuel solenoid and fuel pump must be checked separately to determine defective part.
Cranks but won't start	Faulty ignition	Check to see if points open and close during cranking. If they do not open and close, adjust and set points. Plug and plug wires must be in good condition. Voltage at ignition coil negative terminal (-) must alternate from +12 volts to zero volts as points open and close during engine cranking.
Cranks but won't start	Inoperative choke	With engine not running, check choke vane movement by pushing choke lever arm. Choke must be in closed position with cold engine and must be free to move against bimetal spring. As engine warms up, bimetal spring relaxes and allows choke vane to open fully. The lever will pulsate as engine warms up. See ADJUSTMENT section.
Unit starts, but stops immediately after releasing start switch S1	Low oil level	Check oil level. If low or empty, refill to proper level.
Unit runs then stops	Low oil level	Check oil level. If low or empty, refill to proper level.
Unit runs then stops	Stuck choke	See 3 above (CRANKS BUT WON'T START): With engine not running, check choke vane movement by pushing choke lever arm. Choke must be in closed position with cold engine and must be free to move against bimetal spring. As engine warms up, bimetal spring relaxes and allows choke vane to open fully. The lever will pulsate as engine warms up. See ADJUSTMENT section.
Unit runs but surges	Governor not adjusted properly	Readjust governor.
Unit stops	Faulty ignition	See 4 above (CRANKS BUT WON'T START): Check to see if points open and close during cranking. If they do not open and close, adjust and set points. Plug and plug wires must be in good condition. Voltage at ignition coil negative terminal (-) must alternate from +12 volts to zero volts as points open and close during engine cranking.
Unit stops	Out of fuel	Refill fuel tank.
Unit stops	Low oil level	Check oil level. If low or empty, refill to proper level.
Remote running time meter or generator lamp inoperative	Blown fuse (F1)	Replace F1 fuse on control.
Engine does not crank	Engine cranks at set but not at remote start panel; remote start control wires not continuous between generator set and remote start panel; control wires not sized large enough to avoid excessive voltage drop	Replace control wires with correct wire gauge
Engine does not crank	Battery not fully charged or terminal connections not clean and tight	Recharge battery and clean and tighten all terminal connections
Engine does not crank	No battery voltage present between control terminals 3 and 5 when switch S1 is pushed to START (or when remote start switch is pushed to START); fuse F1 open	Replace start-stop switch S1 or remote start switch as required
Engine does not crank	No battery voltage present between K1 start solenoid terminal S1 and ground when start-stop switch S1 is pushed to START	Replace K1 start solenoid
Engine does not crank	Generator brushes, commutator, DC armature windings, or field windings faulty	Check generator brushes, commutator, DC armature windings, and field windings. See Generator Service Procedures section
Engine cranks but does not start	No battery voltage present between control terminal 6 and ground when start-stop switch S1 is pushed to START	Check all solenoid terminal connections. If OK, replace K1 start solenoid on BF (Spec A) or K2 ignition relay on BF (Spec B) and NH (Spec J)
Engine cranks but does not start	Fuse F2 open	Replace fuse F2
Engine cranks but does not start	Low engine oil level; low oil pressure switch operation	Check engine oil level and add if necessary. If oil level is OK, remove wire lead from low oil pressure switch and push start-stop switch S1 to START. Check low oil pressure switch operation. Switch should open when oil pressure builds up and close when oil pressure drops. Replace if necessary
Engine cranks but does not start	Low oil pressure lead wire grounded	Check low oil pressure lead wire for grounding and repair or replace as necessary
Engine cranks but does not start	High air temperature switch operation	Remove wire lead from high air temperature switch and push start-stop switch S1 to START (do not let lead ground while disconnected). Check high air temperature switch operation. Switch should be open unless engine overheats. Replace if necessary
Engine cranks but does not start	High air temperature lead wire grounded	Check high air temperature lead wire for grounding and repair or replace as necessary
Engine cranks but does not start	Fuel pump does not operate or fuel solenoid does not open during cranking; fuel pump lead wire connections	Check the fuel pump lead wire connections and repair or replace as necessary. Refer to the Fuel System section for testing and service procedures
Engine cranks but does not start	Choke not closed	Refer to the Ignition System section for testing and service procedures
Engine starts but stops when start switch is released	No DC voltage output from the generator between control terminal 6 and ground while cranking	Check R1 resistor and connections. If OK, check brushes, commutator, armature windings, and field windings. Refer to Generator Service Procedures section
Engine is running - then stops	Low fuel level or low oil level	Check for low fuel level or low oil level. Refill with fuel or oil as required
Engine is running - then stops	Low oil pressure, low oil pressure switch malfunction, overheating, or high air temperature switch malfunction	See sections B5 through B11 for the troubleshooting procedures
Engine is running - then stops	No battery voltage present between control terminal 6 and ground when start-stop switch S1 is pushed to START	Check for an open R1 resistor or for open connections between R1 and fuse F2. If OK, check generator brushes, commutator, DC armature windings, and field windings. See Generator Service Procedures section
Engine is running - then stops	Fuse F2 open	Replace fuse F2
Low battery - no charge rate	Battery charger does not show a normal charge rate of 1 to 1-1/2 amps	Check condition of battery. Generator set charger will not recharge a battery that is in a very low state of charge. Use a separate battery charger to bring battery up to full charge
Low battery - no charge rate	Wire connection between CR1 diode, K1 start solenoid, R2 resistor, and battery	Check wire connection between CR1 diode, K1 start solenoid, R2 resistor, and battery
Low battery - no charge rate	CR1 diode shorted or open	Replace defective CR1 diode
Low battery - no charge rate	Defective R2 resistor	Replace R2 resistor
Running time meter inoperative	Wires between control terminal 6 and battery positive terminal for running time meter, or ground wire and connection to meter	Check wires between control terminal 6 and battery positive terminal for running time meter, and ground wire and connection to meter
Running time meter inoperative	Defective running time meter	Replace defective running time meter
Battery condition meter inoperative	Fuse F1 open	Replace fuse F1
Battery condition meter inoperative	Wire connections between remote connector plug terminal 5 (or TB1-5 where applicable) and the battery condition meter and between the battery condition meter and ground	Check wire connections between remote connector plug terminal 5 (or TB1-5 where applicable) and the battery condition meter and between the battery condition meter and ground
Battery condition meter inoperative	Voltmeter does not read battery voltage minus 10 volts between battery charge meter positive terminal and ground; defective zener diode	Replace defective zener diode
Battery condition meter inoperative	Defective battery condition meter	Replace defective battery condition meter
Engine does not crank	No battery voltage present between remote connector plug terminals 3 and 5 (between terminals TB1-3 and TB1-5 where applicable) when switch S1 is pushed to START (or when remote start switch is pushed to START); fuse F1 open	Replace start-stop switch S1 or remote start switch as required
Engine does not crank	No battery voltage present between ground and K3 relay terminal 87A; fuse F2 open; run ignition relay K3 contacts 87A-30 not closed	Replace run ignition relay K3. Replace fuse F2
Engine does not crank	Generator brushes, commutator, DC armature windings, or DC field windings faulty	Check generator brushes, commutator, DC armature windings, and DC field windings. See Generator Service Procedures section
Engine cranks but does not start	No battery voltage present between remote connector plug terminal 6 and ground when start-stop switch is pushed to START; terminal connections on K2 crank ignition relay	Check all terminal connections on K2 crank ignition relay. If OK, replace K2 relay
Engine cranks but does not start	Fuel pump does not operate during cranking; fuel pump lead wire connections	Check the fuel pump lead wire connections and repair or replace as necessary. Refer to the Fuel System section for testing and service procedures
Engine starts but stops when start switch is released	Incorrect oil level	Add oil as required
Engine starts but stops when start switch is released	Low oil pressure switch operation	Disconnect the low oil pressure switch lead wire, ground the end connected to the control, and push the start-stop switch to START. Check low oil pressure switch operation. Switch should close when oil pressure builds up and open when oil pressure drops. Replace if necessary
Engine starts but stops when start switch is released	Low oil pressure lead wire open circuit	Check low oil pressure lead wire for an open circuit and repair or replace as necessary. Reconnect oil pressure lead wire and push start-stop switch to START
Engine starts but stops when start switch is released	Wire connections between R1 resistor and K3 run ignition relay and between R1 resistor and K1 start solenoid; R1 resistor resistance not OK	Check wire connections and resistance of R1 resistor (see Parts Catalog for resistance value). Replace R1 resistor
Engine starts but stops when start switch is released	No voltage at remote connector plug terminal 6 and ground (or TB1-6 and ground where applicable) when start-stop switch S1 is pushed to START; fuse F1 open	Connect a jumper wire between remote connector plug terminals 5 and 6 (or TB1-5 and TB1-6 where applicable) and push start-stop switch S1 to START. Replace K3 run relay
Engine starts but stops when start switch is released	Generator brushes, commutator, DC armature, or field windings faulty	Check generator brushes, commutator, DC armature, and field windings. Refer to Generator Service Procedures section
Engine is running - then stops	Low oil pressure or low oil pressure switch malfunction	See sections C3, C4, and C5 for troubleshooting procedures
Engine is running - then stops	No battery voltage present between remote connector plug terminal 6 and ground (or TB1-6 and ground if applicable) when start-stop switch S1 is pushed to START; fuse F2 open	Replace fuse F2
Engine is running - then stops	Open R1 resistor, open connection between K3 run ignition relay and R1 resistor, or defective K3 run ignition relay	See sections C6 through C11 for troubleshooting procedures
No AC output voltage	Blown fuse or circuit breaker (if used)	Look for cause and repair. Then replace fuse or reset circuit breaker
No AC output voltage	Disconnected wire or lead on brushes	Reconnect wire or wires
No AC output voltage	Brushes not making contact with slip rings	Check brush springs for free movement or brushes which may be excessively worn
No AC output voltage	Open, grounded or short circuit in field or armature winding	Test for open, grounded, or shorted windings. Repair or replace as necessary
Low AC output voltage	External short circuit on line	Locate and eliminate short circuit problem
Low AC output voltage	Generator overloaded	Remove part of load
Low AC output voltage	Shorted or grounded circuit in field or armature winding	Test for open, grounded, or shorted windings. Repair or replace as necessary
Low AC output voltage	Engine not running properly causing generator to slow down	Refer to Engine Troubleshooting Guide
Noisy generator	Defective bearing in end bell	Replace bearing
Noisy generator	Brush rig loose	Retorque
Noisy generator	Armature and field frame rubbing together	Check for generator misalignment and for varnish lumps between armature and field. Repair as necessary
Noisy generator	Raised commutator bar or high mica condition	Turn commutator on lathe; undercut mica insulation
Generator overheats	Generator overloaded	Remove part of load
Generator overheats	Windings and parts covered with oil or dirt	Clean generator. Take necessary steps to allow for proper cooling
Generator overheats	Air intake restricted or incoming air too hot

Fault codes and symptom→remedy steps extracted from this engine’s service/owner documentation; verify against the official manual before servicing. Showing the first 80 of 90 entries.

Common problems

Unsatisfactory or erratic operation of the generator
Cause: Improper governor operation causing over, under, and fluctuating engine speeds
Fix: Check the entire electrical system with a continuity tester or ohmmeter
Plugged or clogged cooling fins
Fix: Check and clean cooling fins at least every 50 hours. Remove any dust, dirt or oil which may have accumulated. Plugged or clogged cooling fins can cause overheating and engine damage.
Overheating
Cause: Plugged or clogged cooling fins
Fix: Check and clean cooling fins at least every 50 hours. Remove any dust, dirt or oil which may have accumulated.
Loss of power, flooding, hard starting and overheating
Cause: Air cleaner becomes too dirty
Fix: Clean the air cleaner every 25 hours of operation or more often under extremely dusty conditions. Remove, clean and oil the paper element and polyurethane pre-cleaner as needed.
Loss of auxiliary power
Fix: 1. Press the reset button(s) on the circuit breaker(s). If the button appears to reset, start the engine to determine whether auxiliary power is now available. 2. Measure the load being drawn. Total load should not exceed 5KW. Check appliance, tool, or cord plugs for shorts.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan BF.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Air Filter Element (3 inch height)		140-1216		Amazon eBay $14.99 NAPA $16.72
Fuel filter			—
Oil filter			122-0338	100 hr	Amazon eBay $13.57
Battery	This cranking motor requires no oiling but when it is dismantled, lubrication should be done as instructed in Disassembly Section.		—
Belt			—
Coolant filter			—
Oil	API SE or SE/CC	2.13 L	—	50 hr
Spark plug	0.025 inch gap		167-0241	100 hr	Amazon Amazon eBay $34.86 eBay $17.97

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

Repair guides

Step-by-step procedures compiled from the Cummins/Onan service manual for this engine.

Service intervals

Item	Action	Hours	Notes
Air Cleaner	Clean	25 hr	Clean and oil polyurethane pre-cleaner every 25 hours; more often in extremely dusty conditions
Crankcase Oil	Drain	25 hr	First service — Drain initial oil fill at 25 operating hours
Cooling Fins	Clean	50 hr	Check and clean cooling fins at least every 50 hours
Crankcase Oil	Change	50 hr	Change every 50 operating hours when engine is warm
Spark Plugs	Check	100 hr	Check, clean and reset every 100 operating hours
Breaker Points	Change	200 hr	Change every 200 hours of operation
Valve Clearance	Adjust	400 hr	Check and adjust at least every 400 operating hours or when poor performance is noticed
Generator Brushes, Collector Rings and Bearing	Inspect	500 hr	Periodic inspection every 500 hours

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	Camshaft Bearing to Camshaft	0.0015-0.0030	in	at 70°F
Clearance	Camshaft Center Pin Extension	3/4	in	from end of camshaft
Clearance	Camshaft End Play	0.003	in	minimum
Clearance	Carburetor Float Clearance	1/8	in	float resting lightly against needle and seat
Clearance	Connecting Rod End Play	0.002-0.016	in	at 70°F
Clearance	Crankshaft End Play	0.006-0.012	in	at 70°F
Clearance	Crankshaft Main Bearing Journal - Standard Size	1.9992-2.0000	in	at 70°F
Clearance	Crankshaft Main Bearing Journal to Bearing Clearance	0.0025-0.0038	in	at 70°F
Clearance	Crankshaft Rod Bearing Journal - Standard Size	1.6252-1.6260	in	at 70°F
Clearance	Crankshaft Rod Journal to Rod Bearing	0.0020-0.0033	in	at 70°F
Clearance	Cylinder Bore - Standard Size	3.1245-3.1255	in	at 70°F
Clearance	Cylinder Out of Round Maximum	0.002	in	rebore and hone if exceeded
Clearance	Cylinder Taper Maximum	0.005	in	rebore and hone if exceeded
Clearance	Gear Cover Oil Seal Depth	31/32	in	from mounting face of cover
Clearance	Generator Brush Wear Limit	5/16	in	replace when worn to this length or less
Clearance	Governor Cup Travel	7/32	in	in and out travel
Clearance	Governor Linkage Clearance	1/32	in	engine stopped; tension on governor spring
Clearance	Oil Pump Gear Backlash	0.002-0.005	in	at 70°F
Clearance	Piston Clearance in Cylinder	0.001-0.003	in	at 70°F; measured .10 below oil control ring, 90° from pin
Clearance	Piston Pin in Piston	0.0002-0.0004	in	at 70°F
Clearance	Piston Pin in Rod	0.0002-0.0007	in	at 70°F
Clearance	Piston Ring Gap in Cylinder	0.010-0.020	in	at 70°F
Clearance	Piston Ring Side Clearance (Top Compression Ring)	0.008	in	replace piston when reached
Clearance	Roll Pin Protrusion	3/4	in	from gear cover mounting surface
Clearance	Starter Brush Wear Limit	.35	in	replace when worn to less than this length
Clearance	Tappet to Cylinder Block Clearance	0.0015-0.0030	in	at 70°F
Clearance	Throttle Stop Screw Clearance	1/32	in	no load
Clearance	Timing Gear Backlash	0.002-0.003	in	at 70°F
Clearance	Valve Seat Width	1/32-1/8	in
Clearance	Valve Stem to Guide - Exhaust	0.0035-0.0040	in	at 70°F
Clearance	Valve Stem to Guide - Intake	0.0010-0.0025	in	at 70°F
Gap	Breaker Point Gap	.025	in	Full Separation
Gap	Spark Plug Gap	.025	in
Other	Ignition Timing	21° BTC	° BTC	HOT
Other	Ignition Timing	25° BTC	° BTC	COLD
Other	Starter Maximum Current Draw	32	Amperes	no-load test
Other	Valve Face Angle	44	°
Other	Valve Seat Angle	45	°
Speed	Idle Speed	2200-2300	rpm
Speed	Maximum Full Load Speed	3600	rpm	continuous operation
Speed	Starter No-Load Test RPM	4800-6000	rpm
Torque	Connecting Rod Bolt	14-16	ft-lb
Torque	Cylinder Head Stud Nuts	14-16	ft-lb	Cold
Torque	Flywheel Cap Screw	35-40	ft-lb
Torque	Gearcase Cover	8-10	ft-lb
Torque	Manifold Mounting Stud Nuts	6-9	ft-lb
Torque	Oil Base	18-23	ft-lb
Torque	Oil Pump	7-9	ft-lb
Torque	Rear Bearing Plate Screws	25-27	ft-lb
Torque	Spark Plug	15-20	ft-lb
Torque	Starter Mounting Bolts	18-20	ft-lb
Torque	Stud and Nuts	8-10	ft-lb
Valve clearance	Exhaust Valve Tappet Adjustment	0.012-0.014	in	Cold
Valve clearance	Intake Valve Tappet Adjustment	0.007-0.009	in	Cold

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	5W30 or 10W	API SE or SE/CC		For temperatures below 30°F
Engine oil	SAE 30	API SE or SE/CC	2.0 U.S. quarts	For temperatures above 30°F; use 4 pints (4-1/2 if equipped with oil filter)
Fuel	Regular grade automotive gasoline		5.6 U.S. gallons	Nonleaded or regular grade; do not use highly leaded premium. Tank capacity 5.6 U.S. gallons.

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

Procedures

Carburetor Adjustment adjustment

⚠ Damage will result if needle is forced against its seat.

Run engine to normal operating temperature.
Make idle adjustment under no load.
Open main jet until engine runs smooth under acceleration with no load.
Open about 1/4 turn further if needed for sudden load.
Set throttle stop screw to 1/32 in clearance.
If hunting, open main needle a little more; do not exceed 1/2 turn beyond maximum power.

Carburetor Float Adjustment adjustment

Invert float assembly and casting.
With float resting lightly against needle and seat, clearance between bowl cover gasket and free end of float should be 1/8 in.
If reset needed, bend float tangs near pin to obtain 1/8 in clearance.

Governor Adjustment adjustment

Tools: tachometer

Adjust carburetor main jet for best full load mixture.
Adjust carburetor idle needle with no load.
Adjust length of governor linkage.
Check governor linkage and throttle shaft for binding or looseness.
Adjust governor spring tension for nominal no-load speed.
Check rpm drop from no-load to full-load and adjust sensitivity.
Recheck speed adjustment.
Set carburetor throttle stop screw.

Tappet Adjustment adjustment

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

Remove parts to gain access to valve tappets.
Remove spark plugs to ease turning crankshaft.
Rotate flywheel until left intake valve opens and closes; align TC mark. Both valves closed.
Intake: .007 in gauge should pass freely; .009 in should not.
Exhaust: .012 in gauge should pass freely; .014 in should not.
Use 7/16 in open end wrench to turn adjusting screw; hold tappet with 9/16 in wrench.
For right cylinder, turn engine one revolution and align TC mark; repeat.
Replace all parts and tighten screws; torque manifold bolts.

Breaker Points Replacement (Cold Setting) maintenance

Tools: screwdriver, flat thickness gauge

Remove control box cover screws and cover.
Disconnect wire from control box to ignition coil; remove front panel.
Remove breaker box cover.
Remove spark plugs to rotate crankshaft by hand.
Remove points mounting nut and disconnect leads.
Replace condenser and points.
Rotate crankshaft clockwise to align 25° BTC mark; turn 1/4 turn more.
Set point gap to .025 in; tighten nut and recheck.

Generator Brush Inspection maintenance

⚠ Do not use a substitute brush.

Remove generator fan guard and fan.
Detach brush guide by removing end bell screws.
Tag leads 5 and 6; unscrew brush retaining screws and pull brushes.
Inspect brush faces for smooth coppery cast; no deep grooves.
Replace brushes if worn to less than 5/16 in length.
Work brushes up and down in guide to ensure no binding.

Out-of-Service Protection (More Than 30 Days) maintenance

Run engine until thoroughly warm.
Turn off fuel supply and run until engine stops.
Drain oil while warm; refill and attach warning tag.
Remove spark plugs; pour 1 ounce rust inhibitor or SAE50 oil into each cylinder; crank and reinstall plugs.
Service air cleaner.
Clean governor linkage and wrap with clean cloth.
Plug exhaust outlets; wipe unit and coat rustable parts.
Disconnect battery; provide suitable cover.

Electric Starting operation

⚠ Do not touch welder terminals while operating.

Adjust choke according to temperature conditions.
Move ignition switch to ON.
Push START switch firmly. If unit does not start within a few seconds, release and wait.
If unit does not start after two attempts, open choke and repeat starting sequence.

Fuel Pump Disassembly repair

⚠ Use care when reassembling; all parts must be perfectly aligned.

Remove vacuum line and fuel line.
Remove two fuel pump attaching screws.
Grasp pump and carefully pull apart.
Diaphragm, plunger, return spring, pump body and gaskets will be loose.
Internal parts are available in a repair kit.

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 oil capacity of the Cummins/Onan BF?

The Cummins/Onan BF engine has an oil capacity of 2.0 liters.

What is the displacement of the Cummins/Onan BF?

The Cummins/Onan BF has a displacement of 0.66 liters with 2 cylinders.

Which generators use the Cummins/Onan BF engine?

The Cummins/Onan BF powers 2 generator set(s), including Cummins/Onan 5.0BF - 34 3E, Cummins/Onan S.OSF - 34 IE.

How often should you change the oil in the Cummins/Onan BF?

The Cummins/Onan BF oil change interval is every 50 hours.

What is a common problem with the Cummins/Onan BF engine?

A commonly reported issue on generators using the Cummins/Onan BF is: Unsatisfactory or erratic operation of the generator. Cause: Improper governor operation causing over, under, and fluctuating engine speeds. Fix: Check the entire electrical system with a continuity tester or ohmmeter.