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Engine lacks power

Why generators show "engine lacks power": 51 documented causes and brand-specific fixes, compiled across 17 manufacturers.

Dirty air filter

3 brands
  • Champion: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.
  • Dayton: See “Finding Total Wattage” on Page 12, Replace air filter
  • Generac: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.; 1. Reduce load (see “Generator Loads”). 2. Clean or replace air filter. 3. Replace fuel filter. 4. Contact Authorized Service Facility.

Dirty air cleaner

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.; 1a. Adjust breaker point gap 1b. Reset spark plug gap 2. Replace air cleaner 3a. Clean fuel filter 3b. Test fuel pump and repair or replace if faulty 4a. Adjust carburetor main and idle adjustment screws 4b. Adjust carburetor float level 4c. Disassemble carburetor and clean all internal passages
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Exhaust system blocked or restricted

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.; 1a. Adjust distributor. 1b. Reset spark plug gap. 2. Test fuel pump and replace if faulty. 3a. Adjust carburetor main and idle adjustment screws. 3b. Adjust carburetor float level. 3c. Disassemble carburetor and clean all internal passages. 4. Locate and remove cause of blockage. 5. Adjust valve clearance (see Section 9 - Engine Block Assembly) 6. See Section 9 - Engine Block Assembly
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Excessive engine wear or damage to engine

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.; 1a. Adjust distributor. 1b. Reset spark plug gap. 2. Test fuel pump and replace if faulty. 3a. Adjust carburetor main and idle adjustment screws. 3b. Adjust carburetor float level. 3c. Disassemble carburetor and clean all internal passages. 4. Locate and remove cause of blockage. 5. Adjust valve clearance (see Section 9 - Engine Block Assembly) 6. See Section 9 - Engine Block Assembly
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Faulty ignition due to incorrect spark plug gap

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Restricted fuel flow due to: a. Plugged fuel filter. b. Faulty fuel pump

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Incorrect fuel mixture due to: a. incorrectly adjusted fuel mixture screws. b. incorrect float level. c. dirt in carburetor. d. vacuum leak

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Incorrect valve tappet clearance

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Carburetor air preheater set incorrectly

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

Combustion chamber deposits

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

No-load speed set too low: excessive governor droop

3 brands
  • Bgm/Nhm: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Cummins/Onan: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.
  • Marquis: 1. Reset spark plug gap. 2. Replace air cleaner. 3a. Replace fuel filter. 3b. Test fuel pump and repair or replace if faulty. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. Replace fuel filter. 4d. Repair vacuum leak. 5. Locate and remove cause of blockage. 6. Adjust valve tappets (see Engine Block Assembly section). 7. See Engine Block Assembly section. 8. In hot weather, place air preheater in summer position. 9. Clean combustion chamber. 10. Adjust (mechanical) governor.

(1) Air-to-carburetor line clogged. Remove restriction. (2) Muffler choked. Clean or replace muffler.

2 brands

Load is too high

2 brands
  • Champion: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.
  • Generac: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.; 1. Reduce load (see “Generator Loads”). 2. Clean or replace air filter. 3. Replace fuel filter. 4. Contact Authorized Service Facility.

Engine needs to be serviced

2 brands
  • Champion: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.
  • Generac: 1. Reduce load (see Know Generator Limits). 2. Clean or replace air filter. 3. Contact IASD.; 1. Reduce load (see “Generator Loads”). 2. Clean or replace air filter. 3. Replace fuel filter. 4. Contact Authorized Service Facility.

Fuel strainer and fuel filter clogged, Air cleaner clogged

2 brands
  • EMC: -Service the fuel strainer and fuel filter (pars. 50 and 51). -Service air cleaner (par. 28).
  • Libby Welding: -Service the fuel strainer and fuel filter (pars. 50 and 51). -Service air cleaner (par. 28).

Affected models

Cummins/Onan 10HQ-150RCummins/Onan 15HQ-150RCummins/Onan BGDCummins/Onan KCummins/Onan NHCummins/Onan P4500

Related fault codes

Cummins/Onan fault codesGenerac fault codes

Common questions

What causes engine lacks power?

Common causes, most-reported first: Dirty air filter; Dirty air cleaner; Exhaust system blocked or restricted; Excessive engine wear or damage to engine; Faulty ignition due to incorrect spark plug gap; Restricted fuel flow due to: a. Plugged fuel filter. b. Faulty fuel pump.