D.W.Onan ACK Engine Engine

• Spark plug gap too narrow
  Fix: Adjust, clean if needed.
• SHARP METALLIC THUD, ESPECIALLY WHEN COLD ENGINE FIRST STARTED
  Cause: Low oil supply
  Fix: Add oil. Change if necessary.
• Power drops under heavy load
  Cause: Engine lacks power, Poor compression, Faulty carburetion, Carbon in cylinders or in carburetor venturi, Restricted exhaust line, Governor not properly adjusted, Fuel level in carburetor too low
  Fix: See remedies for engine missing under heavy load., Tighten cylinder heads and spark plugs. If still not corrected, repair engine., Check fuel system. Open choke. Clean air cleaner. Adjust air-fuel mixture for optimum setting., Remove carbon., Clean or increase the size., Adjust governor., Adjust float.
• PINGING SOUND WHEN ENGINE IS SUDDENLY OR HEAVILY LOADED
  Cause: Spark advanced too far
  Fix: Reset breaker points or retime ignition.
• Low oil pressure
  Cause: Oil too light or diluted from leaking pump diaphragm, Oil level too low, Oil relief valve not seating, Badly worn engine, Badly worn oil pump, Sludge on oil screen, Defective oil pressure gauge
  Fix: Drain, refill with proper oil. Replace fuel pump., Add oil., Remove and clean, or replace., Overhaul., Replace., Remove and clean., Replace.
• Engine will not start when cranked
  Cause: No fuel, Fuel system restricted, Cylinders flooded, Faulty ignition, Poor compression
  Fix: Replenish., Open., Crank with plugs removed., Service spark plugs and breaker points. Retime if necessary., Repair engine.
• ENGINE STOPS UNEXPECTEDLY
  Cause: Fuel tank empty
  Fix: Refill. Open valve.
• Engine misfires at light load
  Cause: Intake air leak, Faulty ignition, Poor compression
  Fix: Tighten or replace gaskets, Service ignition, Tighten cylinder heads and spark plugs. Repair engine.
• Engine misfires at heavy load
  Cause: Defective spark plug or cable, Faulty ignition, Clogged carburetor or fuel system
  Fix: Replace spark plug or cable, Service ignitions system, Clean carburetor or fuel system.
• Engine misfires at all speeds
  Cause: Worn spark plug, Sticking valves, Broken valve spring, Defective ignition wires, Defective or improperly adjusted points
  Fix: Clean and adjust or replace with correct plug, Grind, or replace, Replace, Replace, Adjust or replace breaker points.
• Engine cranks too stiffly
  Cause: Congealed lubricating oil, Engine stuck
  Fix: Change to recommended oil., Disassemble and repair.
• DULL METALLIC THUD, IF NOT BAD MAY DISAPPEAR AFTER FEW MINUTES OPERATION. IF BAD, INCREASES WITH LOAD
  Cause: Loose crankshait bearing
  Fix: Replace, unless one of the next two remedies permanently corrects the trouble.

Compiled from service & troubleshooting documentation across every generator set that uses the D.W.Onan ACK.

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