Spark Plug: Inspection

Check the electrodes and inner and outer ceramic insulator of plugs, noting the type of deposits and the degree of electrode erosion.

Fig 1: Identifying Spark Plug Damage Locations

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1. Normal:

   Brown to grayish-tan deposits and slight electrode wear indicate correct spark plug heat range.

   Fig 2: Identifying Normal Spark Plug

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2. Carbon fouled:

   Dry fluffy carbon deposits on insulator and electrode are mostly caused by slow speed driving in the city, weak ignition, too rich fuel mixture and dirty air cleaner.

   Fig 3: Identifying Carbon Fouled Spark Plug

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3. Oil fouled:

   Wet black deposits show oil entrance into the combustion chamber through worn rings and pistons or excessive clearance between valve guides and stems.

   Fig 4: Identifying Oil Fouled Spark Plug

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4. Overheating:

   White or light gray insulator with black or brown spots and bluish burnt electrodes indicate engine overheating. Moreover, the appearance results from incorrect ignition timing, loose spark plugs, wrong selection of fuel, hotter range plug, etc.

   Fig 5: Identifying Overheated Spark Plug

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