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: Checking Spark Plug Gap

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Spark plug gap
Carbon accumulation or wear
Cracks
Damage
Damaged gasket

Normal:

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

Fig 2: Checking Spark Plug Gap For Carbon Accumulation Or Wear

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Carbon fouled:

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

Fig 3: Checking For Carbon Fouled

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Oil fouled:

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

Fig 4: Checking For Oil Fouled

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Overheating:

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

Fig 5: Checking For Overheating

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