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Cummins/Onan MDSL Engine

The Cummins/Onan MDSL is a 0.27 L 1-cylinder engine. It powers generator sets from Cummins/Onan, including the MDSL Series.

Also known as: Onan MDSL

0.27 L

Displacement

Cylinders

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

Engine specifications

Displacement	0.27 L
Cylinders	1

Generator sets powered by this engine

Cummins/Onan MDSL Series

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

Common problems

Excessive oil consumption, light blue smoky exhaust
Cause: Poor compression. Usually due to worn piston, rings, or cylinder
Fix: Refinish cylinder. Install oversize piston and rings
Low oil pressure
Cause: Defective oil pressure gauge
Fix: Replace
Engine cranks slowly or will not crank
Cause: Defective or discharged battery
Fix: Replace or recharge battery
Voltage drops under heavy load
Cause: Engine lacks power
Fix: See Engine Misfires at Heavy Loads
Engine will not start when cranked
Cause: Air in fuel system
Fix: Bleed the fuel system
Engine stops unexpectedly
Cause: Empty fuel tank
Fix: Refill fuel tank as often as needed to prevent running out of fuel
Engine overheating
Cause: Improper lubrication
Fix: See Low Oil Pressure
Black, smoky exhaust, excessive fuel consumption, possible lack of power under load
Cause: Generator overloaded. Black smoky exhaust normal condition with overload
Fix: Reduce load to within rated capacity. If smoky condition does not clear up, stop the unit at once and check for further trouble. Serious damage may result if trouble is not corrected
Voltage unsteady but engine not misfiring
Cause: Speed too low
Fix: Adjust governor to correct speed
Light pounding knock (NOTE: DO NOT CONFUSE WITH NORMAL KNOCK FROM FIRING OF FUEL)
Cause: Loose connecting rod
Fix: Adjust clearance or re
High oil pressure
Cause: Defective oil pressure gauge
Fix: Replace
Engine cranks too stiffly
Cause: Load not disconnected from unit
Fix: Disconnect load from unit when starting
Generator overheating
Cause: Short in load circuit
Fix: Correct short circuit
Engine misfires at light load
Cause: Faulty injection
Fix: Dirty fuel is main cause. Use only recommended fuels
Engine misfires at heavy load
Cause: Faulty ignition
Fix: Dirty fuel is main cause. Use only recommended fuels

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

Maintenance schedule

Item	Spec	Part #	Interval
Air filter	clean the air cleaner at intervals and in the manner as experience shows necessary	—
Fuel filter		—
Oil filter		—	100 hr
Battery	check the level of the fluid in the starting batteries. Add distilled water, if necessary	—
Belt		—
Coolant		—
Oil	Change oil (check level daily)	—
Spark plug		—

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
Crankcase oil level	Check	8 hr		Daily or end of each 8 hours; fill to full mark with SAE No. 30 oil (API Service DG) or as recommended for low temperature.
Fuel supply	Check	8 hr		Daily or end of each 8 hours of running time, whichever occurs first.
Plant cleanliness	Clean	8 hr		Daily service.
Governor linkage	Lubricate	50 hr		Weekly or end of each 50 hours; powdered graphite on ball joint, SAE No. 30 oil on other joints.
Starting battery electrolyte	Check	50 hr		Weekly or end of each 50 hours; add distilled water to 3/8 inch above separators.
Valve clearance	Check	50 hr		First service — Check at end of first 50 hours; check only as required thereafter.
Crankcase oil	Change	100 hr		Every 100 hours unless sludge or condensation forms during cold weather; then change more often.
Oil filter cartridge	Replace	100 hr		Replace each time crankcase oil is changed.
Anti-flicker breaker points	Check	200 hr	1 mo	Monthly or end of each 200 hours; clean and reset gap to .025 inch if necessary.
Exhaust connections	Inspect	200 hr	1 mo	Monthly or end of each 200 hours; tighten or replace parts requiring it.
Generator brushes	Inspect	200 hr	1 mo	Monthly or end of each 200 hours; replace brushes worn to 5/8 inch.
Exhaust system carbon deposits	Inspect	1200 hr	6 mo	Semi-yearly or after each 1200 hours; remove carbon especially if consistently light loaded.
Generator brush rig	Check	1200 hr	6 mo	Semi-yearly or after each 1200 hours; ensure it has not shifted from neutral position.
Primary fuel filter bowl	Drain	1200 hr	6 mo	Empty water and sediment each 6 months or 1200 operating hours.
Secondary fuel filter	Drain		6 mo	Drain sludge each 6 months or oftener; excessive sludge indicates new element should be installed.

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 clearance	0.001 to 0.003	in
Clearance	Camshaft endplay	0.003	in
Clearance	Compression release adjusting screw - with release at running position	0.027	in	release at running position
Clearance	Connecting rod bearing clearance - forged steel rod with precision type bearings, measured parallel with length of rod	0.001 to 0.003	in
Clearance	Connecting rod endplay	0.002 to 0.011	in
Clearance	Crankshaft endplay	0.010 to 0.015	in
Clearance	Crankshaft main bearing clearance	0.0025 to 0.004	in
Clearance	Exhaust valve head to face of cylinder head	0.030	in	maintain by grinding new seat
Clearance	Governor arm stop to throttle lever stop screw	1/32	in	engine stopped, wide open throttle
Clearance	Governor cup in and out travel	7/32	in
Clearance	Oil pump gear backlash	0.003 to 0.005	in
Clearance	Piston clearance in cylinder at bottom of skirt	0.0035 to 0.0055	in
Clearance	Piston pin clearance in piston - tap fit	0.0000 to 0.0003	in
Clearance	Piston pin clearance in rod bushings	0.0002 to 0.0007	in
Clearance	Throttle stop pin feeler gauge	0.030	in
Clearance	Timing gear backlash	0.001 to 0.006	in
Clearance	Valve rocker arm clearance	0.015	in	cold
Clearance	Valve seat width	3/64 to 1/16	in
Clearance	Valve stem clearance in guide - exhaust	0.003 to 0.0045	in
Clearance	Valve stem clearance in guide - intake	0.0015 to 0.003	in
Clearance	Water pump bushing clearance	0.001 to 0.003	in
Clearance	Water pump bushing installation distance from back side of gear cover	3/4	in
Clearance	Water pump bushing reamed diameter	0.501 to 0.502	in
Clearance	Water pump shaft end play	0.018 to 0.044	in	after installing gear and screw plug
Gap	Anti-flicker breaker point gap - AC units only	0.025	in
Gap	Other compression rings and oil ring gap in cylinder	0.010 to 0.015	in
Gap	Top compression ring gap in cylinder	0.010 to 0.020	in
Injection timing	Injection pump timing	6° BTC - P.O.	degree	port opening
Injection timing	Injection pump timing gauge setting	1.552 ± 0.002	in
Other	Battery electrolyte level above separators	3/8	in
Other	Brush replacement length	5/8	in	replace brushes worn to this length
Other	Camshaft center pin extension	3/4	in
Other	Crankshaft main bearing journal - standard size	2.7495 to 2.7500	in
Other	Crankshaft rod journal - standard size	2.3745 to 2.3750	in
Other	Cylinder bore - standard size	3.5015 to 3.5025	in
Other	Generator armature run-out	0.012	in	maximum
Other	Valve face angle	44	degree
Other	Valve seat angle	45	degree
Pressure	Compression reading	at least 370	pounds per square inch	at 70°F
Pressure	Injection nozzle opening pressure	1750 minus 0, plus 50	lbs.
Pressure	Oil pressure at normal operating temperature	20 to 30	lbs.	normal operating temperature
Pressure	Oil pressure shut-off threshold	10	lbs.	shut off at once if pressure drops to 10 lbs.
Speed	Battery charging plants - full load engine speed	not less than 1750	rpm	full load
Speed	Battery charging plants - no load engine speed	not greater than 1950	rpm	no load
Speed	Maximum no load engine speed - 50 cycle AC plants	1710	rpm	no load
Speed	Maximum no load engine speed - 60 cycle AC plants	1920	rpm	no load
Speed	Maximum speed drop from no load to full load	60	rpm
Speed	Minimum engine speed at full rated generator capacity - 50 cycle AC plants	1500	rpm	full rated generator capacity
Speed	Minimum engine speed at full rated generator capacity - 60 cycle AC plants	1710	rpm	full rated generator capacity
Temperature	High water temperature cut-off switch	200 ± 7	°F
Temperature	Thermostat opening	approximately 150	°F	installation section
Temperature	Thermostat opening	approximately 160	°F
Torque	Injection nozzle mounting screws	15	lb. ft.
Voltage	Maximum no load voltage - 120 V circuits	126	V	no load
Voltage	Maximum no load voltage - 240 V circuits	252	V	no load
Voltage	Minimum voltage at full rated generator capacity - 120 V circuits	110	V	full rated generator capacity
Voltage	Minimum voltage at full rated generator capacity - 240 V circuits	220	V	full rated generator capacity

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
Coolant	clean alkali-free water			If preparation is for below freezing temperatures, use approved antifreeze in proportion recommended by its manufacturer. Drain entire cooling system as soon as engine stops if operated in freezing temperatures unless fresh water system with antifreeze is used.
Engine oil	SAE No. 10W	API Service DG or MS/DG	4.0 quarts	Below 40°F; crankcase capacity excluding filter.
Engine oil	SAE No. 30	API Service DG or MS/DG	4.0 quarts	Above 40°F; crankcase capacity excluding filter. Multiviscosity oils such as 5W-20 or 10W-30 are not recommended, especially at higher temperatures.
Fuel	No. 2 Furnace Oil			Premium Diesel fuels not required. Alternates: No. 1 Furnace Oil (distillate and range oil) and kerosene may be used but one quart SAE No. 30 lubricating oil should be added to each 25 gallons. Specifications: gravity minimum 30 API; viscosity 30.5 to 45 Saybolt Universal at 100°F; cetane number 43 minimum; sulphur 1% maximum by weight; Conradson carbon 0.15% maximum; flash point 130°F minimum or legal minimum; water and sediment none by volume; pour point must be 10° lower than minimum temperature at which fuel oil is to be used.
Grease	powdered graphite			Use on governor linkage ball joint; use light non-gummy oil if powdered graphite is unavailable. Place a drop of SAE No. 30 oil on other governor linkage joints.

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 Valve Tappet and Compression Release Clearance adjustment

Tools: feeler gauges, wrench

Remove top plate from rocker box.
Turn crankshaft clockwise until TC mark aligns with flywheel housing mark on compression stroke.
Place compression release mechanism in disengaged RUN position.
Back away exhaust valve tappet adjusting screw.
Set compression release clearance to 0.027 inch with feeler gauge and tighten lock nut.
Set exhaust valve tappet clearance to 0.015 inch cold with feeler gauge.
Repeat for intake valve, reinstall parts and return release to engaged START position.

Nozzle Adjustment adjustment

⚠ Turning screw out too far may force burning gases into nozzle and foul pintle.

Tools: screwdriver, wrench

Run engine until thoroughly warmed up.
Remove top cover from nozzle holder.
Loosen lock nut just enough to turn adjusting screw.
Turn adjusting screw in one direction and then the other until knock is least noticeable.
Do not turn adjusting screw more than one turn in either direction from original position.
Lock adjusting screw nut and replace cover with copper washer in place.

Timing the Injection Pump to the Engine adjustment

Tools: timing gauge, depth micrometer

Crank engine slowly by hand until PO mark on flywheel aligns with timing hole mark on compression stroke.
Remove fuel injection pump.
Insert wider end of timing gauge into injection pump opening so bottom contacts tappet screw.
Measure setting with depth micrometer; setting should be 1.552 inch plus or minus 0.002 inch.
Remove tappet to adjust and repeat until correct.
Install injection pump with neoprene seal in place and connect fuel lines.
Bleed fuel system.

Connecting Battery - AC Models installation

⚠ Disconnect battery negative cable first before maintenance.

For single 12V battery, connect long cable from positive post to POSITIVE terminal of start solenoid.
Connect other long cable from negative post to NEGATIVE terminal on generating plant.
Short jumper cable is not used with single 12V battery.
For two 6V batteries in series, connect short jumper from negative post of one to positive post of other.
Then make long cable connections as above and ensure all connections tight.

Bleeding the Fuel System maintenance

⚠ If the cam is on the high side the pump will not operate; turn crankshaft one complete revolution to correct.

Loosen the injection pump bleeder plug located just above the throttle arm.
Work the primer lever on the fuel pump until fuel flows freely with no air or foreign gas bubbles.
Tighten the plug, or leave it cracked during the first minutes of operation.
Bleed the secondary fuel filter plug at the top, then tighten it before completing injection pump bleeding.
Leave the primer lever at the in position when through priming.

Cleaning Primary Fuel Filter Strainer Element maintenance

⚠ Have clean hands.

⚠ Do not attempt to wipe dry.

Loosen nuts and carefully separate discs.
Submerge and swish in clean diesel fuel or suitable solvent.
Rinse thoroughly in clean fuel.
Compressed air may be blown in reverse direction of fuel flow; otherwise do not dry element.
Retighten nuts and prevent dirt from entering internal area of discs.

Preparing Unit for Storage maintenance

If lubricating oil is dirty, drain while hot and install new filter element; attach warning tag.
Remove glow plug and pour two tablespoonfuls of rust inhibitor oil or SAE 50 into cylinder.
Slowly crank engine to lubricate cylinder, then stop at top center and replace glow plug.
Wipe exposed parts clean and coat with grease.
Disconnect battery; store away from freezing and give freshening charge about every 40 days.
Clean generator brushes, holders, commutator and collector rings with clean cloth; do not coat with lubricant.
Leave fuel supply and fuel system filled but guard against siphoning.

Priming the Water Pump maintenance

Remove plug in pump inlet fitting.
Add water while cranking engine.
Prime before starting a new engine or whenever cooling system has been drained.

Servicing Water Pump Impeller maintenance

Tools: pliers

Remove pump nameplate and gasket.
Grasp old impeller with pliers and pull straight out.
Avoid damaging brass body.
Remove pump body and gaskets as needed.
Install new impeller and reassemble with gaskets and nameplate.

Starting Electrically - Electric-Starting Plants operation

Place compression release lever in START-STOP position.
Depress both START switch and HEATER switch.
When engine is turning over rapidly, throw compression release lever to RUN position.
Continue depressing both switches until engine comes up to speed.

Starting Electrically - Remote-Starting Plants operation

Disengage decompression release plunger if manually locked IN.
Depress START switch to crank electrically.
Manifold heater and glow plug heat automatically during cranking.
When engine turns fast enough to build oil pressure, decompression release operates.
Continue pressing START until engine comes up to speed.

Starting Manually - Remote-Starting Plants operation

Tools: hand crank

Engage compression release to prevent compression.
Work priming lever on transfer pump a few times and leave lever inward.
Depress heater switch one minute before cranking and hold during cranking.
Engage hand crank and crank the engine.
At top cranking speed, disengage compression release to allow compression and running.
Check lubricating oil pressure immediately.
Do not operate plant unless battery is connected.

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
120	1	2 wire	M1 hot, M2 grounded	Generator output leads terminate in junction box; M1 connects to hot side of fused switch or circuit breaker; M2 connects to grounded side.
120/240	1	3 wire	M1 hot, M2 grounded, M3 hot	Two 120V circuits across M1-M2 and M2-M3; one 240V circuit across M1-M3. Load on each 120V circuit should not exceed 1/2 plant capacity.
240	3	3 wire	M1, M2, M3; none grounded	Single-phase 240V and three-phase 240V obtainable. Reverse any two leads to correct rotation. Use phase sequence indicator for in-phase connections.

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 MDSL?

The Cummins/Onan MDSL has a displacement of 0.27 liters with 1 cylinders.

Which generators use the Cummins/Onan MDSL engine?

The Cummins/Onan MDSL powers 1 generator set(s), including Cummins/Onan MDSL Series.

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

A commonly reported issue on generators using the Cummins/Onan MDSL is: Excessive oil consumption, light blue smoky exhaust. Cause: Poor compression. Usually due to worn piston, rings, or cylinder. Fix: Refinish cylinder. Install oversize piston and rings.