Cummins/Onan J120-S/967F Engine

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

• 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.

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

• Commutator Brush Fitting and Seating
• AC Generator Removal and Disassembly
• Bearing, Fan, and Key Removal from Rotor Assembly
• Stator Winding Removal from Housing
• Voltage Regulator Removal and Installation

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

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