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Soft, black, sooty deposits easily identify this plug condition. This is most often caused by an over-rich, air/fuel mixture.
Check for a sticking choke, clogged air cleaner, or a carburettor problem – float level high, defective needle or seat, etc.

This may also be attributed to weak ignition voltage, an inoperative preheating system (carburettor intake air), or extremely low cylinder compression.
 

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