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Engine misfires at heavy load

Why generators show "engine misfires at heavy load": 17 documented causes and brand-specific fixes, compiled across 10 manufacturers.

Defective spark plug. Faulty ignition. Clogged carburetor. Clogged fuel screen. Defective spark plug cable.

3 brands
  • Cummins/Onan: Replace. Clean, adjust, or replace breaker points, spark plugs, condenser etc., or retime ignition. Clean carburetor. Clean. Reglace.; Replace. Clean, adjust, or replace breaker points, spark plugs, condenser, etc. or retime ignition. Clean carburetor.
  • Onan Electric: Replace. Clean, adjust, or replace breaker points, spark plugs, condenser, etc. or retime ignition. Clean carburetor.
  • Prestolite: Replace. Clean, adjust, or replace breaker points, spark plugs, condenser etc., or retime ignition. Clean carburetor. Clean. Reglace.

Defective spark plug.

2 brands
  • Cummins/Onan: Replace. Faulty ignition. Clean, adjust, or replace breaker points, spark plug, condenser, etc.
  • Doosan: Replace. Faulty ignition. Clean, adjust, or replace breaker points, spark plug, condenser, etc.

Spark plug gap too wide

2 brands
  • Cummins/Onan: Adjust gap; Adjust to .025.
  • generatorpartspro: Adjust gap Clean, adjust, or replace breaker points, plug, condenser, etc. Clean jet Clean Replace

Defective spark plug. Faulty ignition. Clogged carburetor. Clogged fuel screen, Defective spark plug cable.

2 brands
  • Garretson: Replace. Clean, adjust, or replace breaker points, spark plugs, condenser, etc. or retime ignition. Clean carburetor. Clean. Replace.
  • NEC: Replace. Clean, adjust, or replace breaker points, spark plugs, condenser, etc. or retime ignition. Clean carburetor. Clean. Replace.

Faulty carburetion

1 brand
  • Hol-Gar: See Trouble-Shooting Chart 2

Spark plug defective. Faulty ignition. Clogged carburetor. Clogged fuel screen. Defective spark plug cable.

1 brand
  • Cummins/Onan: Replace. Clean, adjust or replace breaker points, plug, condenser, etc. Clean jets. Clean. Replace.

Spark plugs defective.

1 brand
  • Cummins/Onan: Replace.

Spark plugs defective. Faulty ignition.

1 brand
  • Cummins/Onan: Replace. Clean, adjust, or replace breaker points, plugs, condenser, coil, etc., or retime ignition.

Overloaded.

1 brand
  • Doosan: Reduce load.

Spark plug gap too wide. Faulty ignition. Clogged carburetor. Clogged fuel screen.

1 brand
  • Cummins/Onan: Adjust gap. Clean, adjust or replace spark plugs. Clean jet and aajust carb.

Faulty ignition, Clogged carburetor

1 brand
  • Cummins/Onan: Clean, adjust or replace spark plugs, Clean jet and adjust carb

Defective spark plug. Replace. Clean, adjust, or replace breaker points, spark plugs, condensers, etc. or retime ignition. Faulty ignition. Clogged carburetor. Clean carburetor. Clogged fuel screen. Clean. Defective spark plug cable. Replace.

1 brand

Spark plug gap too wide.

1 brand
  • Cummins/Onan: Adjust gap.

Defective spark plug or cable, Faulty ignition, Clogged carburetor or fuel system

1 brand
  • Cummins/Onan: Replace spark plug or cable, Service ignitions system, Clean carburetor or fuel system.

Spark plugs defective.; Faulty ignition.; Clogged carburetor. Clogged fuel screen.; Defective spark plug cables.

1 brand
  • Cummins/Onan: Replace.; Clean, adjust or replace breaker points, plugs, condenser, coil, etc., or retime magneto.; Clean jets.; Clean.

Affected models

Cummins/Onan 05AKCummins/Onan 06AKCummins/Onan 07AKCummins/Onan 102AKCummins/Onan 105AKCummins/Onan 10HQ-150RCummins/Onan 15HQ-150RCummins/Onan BCummins/Onan CCummins/Onan CCCummins/Onan CCK

Related fault codes

Cummins/Onan fault codes

Common questions

What causes engine misfires at heavy load?

Common causes, most-reported first: Defective spark plug. Faulty ignition. Clogged carburetor. Clogged fuel screen. Defective spark plug cable.; Defective spark plug.; Spark plug gap too wide; Defective spark plug. Faulty ignition. Clogged carburetor. Clogged fuel screen, Defective spark plug cable.; Faulty carburetion; Spark plug defective. Faulty ignition. Clogged carburetor. Clogged fuel screen. Defective spark plug cable..