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 carburettor and ignition timing are properly adjusted.
To obtain an accurate reading from a new spark plug:
- Accelerate at full throttle on a straight,
- Push the engine stop button and pull the clutch lever in to release the clutch,
- Coast to a stop,
- Remove the spark plug.
This information is intended to assist you in correctly choosing your spark plugs and determining the performance condition of your engine.
Even plugs which present a good appearance such as those show in Figures 6-24 can quite often be covered with a lead deposit which causes misfiring.
Wet plug firing-ends such as shown in Figures 1-2 are normally attributed to one of the following causes:
- Excessive choking.
- Trouble within the ignition system.
- Oil pumping past worn piston rings and valve guides.
The causes of sooty plugs like those shown in Figures 3, 4 and 5 are usually the result of:
- A plug with a too high heat rating is being used and the plug firing-end does not reach its self-cleaning temperature (above 400-450C) due to light load conditions.
- Use of a too rich air-fuel mixture of richer than 8:1-10:1.
- Trouble within the ignition system.
- Improperly functioning cooling system resulting in excessive cooling.
The firing-end burns illustrated in Figures 25, 26 and 27 may be a result of:
- Too low heat rating, permitting the plug to exceed the highest limit for optimum operating temperature of over 850-1000C due to excessively heavy load operations.
- A too lean air-fuel mixture.
- Ignition timing too over advanced.
- Abnormal combustion such as knocking.
- Cooling system trouble, which causes engine overheating.
Overheating conditions shown in Figures 28 and 29 are due to intense knocking and pre-ignition following situations identical to those in Figures 25, 26 and 27 where increased temperature of the spark plug firing end results in melting of the electrode.
