Spark Plug Visual Gauge

Posted by Bare | Last Updated August 22, 2013

The appearance of the firing end of a spark plug graphically reflects the condition of an engine, the suitability of the spark plug heat rating and whether or not the carburetor and ignition timing are properly adjusted. These pictures can assist you in correctly choosing your spark plugs and determining the performance condition of your engine.

  • #1 and #2 show wet plug firing ends which may be the result of:
    • Excessive choking
    • Trouble in the ignition system
    • Oil pumping past worn piston rings and valve guides
  • #3-5 show sooty plugs which may be the result of:
    • A plug with too high of a heat rating is being used and the plug firing end does not reach it’s self-cleaning temperature (above 400-450°C) due to light load conditions
    • A too rich air/fuel mixture, richer than 8:1 ~ 10:1
    • Trouble in the ignition system
    • Improperly functioning cooling system resulting in excessive cooling
  • #6-24 show plugs which represent a good appearance, but these plugs can still be covered with a lead deposit that causes misfiring
  • #25-27 show firing end burns which may be the result of:
    • A plug with too low of a heat rating is being used and the plug firing end exceeds the limit for optimum operating temperature (above 850-1000°C) due to heavy load conditions
    • A too lean air/fuel mixture
    • Ignition timing too over advanced
    • Abnormal combustion such as knocking
    • Improperly functioning cooling system resulting in engine overheating
  • #28-29 show overheating conditions which may be the result of intense knocking and pre-ignition following situations identical to those in #25-27 where increased temperature of the spark plug firing end results in melting of the electrode
Oil fouled Oil fouled Carbon fouled Too cold
Too cold Cold or rich but OK Cold or rich but OK Cold or rich but OK
Good Good Good Good
Really good The best Best Best
Best Good Good Good
Kinda hot but OK Hot or lean but OK Hot or lean but OK Hot or lean but OK
Too hot or lean
Pre-ignition range
Too hot or lean
Pre-ignition range
Too hot or lean
Pre-ignition range
Too hot or lean
Pre-ignition range
Too hot or lean
Pre-ignition range

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