1. Normal:
Grey-brown deposits, lightly coated core nose. Plugs ideally suited to engine, and engine in good condition. | 2. Oil Fouling:
Wet, oily deposits.
Fault:
Worn bores/piston rings or valve guides; sometimes occurs (temporarily) during running-in period. |
3. Heavy Deposits:
A build up of crusty deposits. Light grey sandy color in appearance.
Fault:
Often caused by worn valve guides, excessive use of upper cylinder lubricant or idling for long periods. | 4. Overheating:
Electrodes have glazed appearance; core nose very white - few deposits.
Fault:
Plug overheating.
Check:
Plug value, ignition timing, fuel octane (too low) and mixture (too lean). |
5. Lead Glazing:
Plug insulator firing tip appears yellow or green/yellow and shiny in appearance.
Fault:
Often caused by incorrect carburation, excessive idling followed by sharp acceleration. Also check ignition timing. | 6. Electrode Damage:
Electrodes burned away. Core nose has burned glazed appearance.
Fault:
Pre-ignition.
Check:
Correct heat range and as for "overheating", above. |
7. Carbon Fouling
Dry, black, sooty deposits.
Fault:
Over-rich fuel mixture.
Check:
Carburettor mixture settings, float level, choke operation, air filter. | 8. Split Core Nose:
(may appear initially as a crack).
Fault:
Detonation or wrong gap setting.
Check:
Ignition timing, cooling system, fuel mixture (too lean). |
9. Worn plugs
Rounded electrodes with small amounts of deposits on the firing end. Normal color. Causes hard starting in damp or cold weather and poor fuel economy.
Fault:
Plugs left to long in the engine. Replace with new plugs of the same heat range.
| 10. Splashed deposits:
Forms after long periods of misfiring.
Fault:
Deposits can loosen when normal temperatures are restored after a tune-up. At high speed the deposits flake off the piston and are thrown against the hot insulator, causing misfiring.
Replace or clean the plugs. |
11. Gap bridging
Deposits lodge between the electrodes.The plug cease to fire resulting in a dead cylinder
Check:
Locate the plug and remove the deposits, check out point 3. | 12. Mechanical damage:
Can be caused by foreign object in the cylinder or the piston striking an incorrect reach (The plug is too long).
Check:
If other cylinders with same plugs are affected then it is an incorrect reach. Change the plugs for the proper type. Otherwise check with endoscope if the foreign object is still there and if there is damage to valves, cylinder liner....etc. Replace the plug. |
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