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Causes of running on Spark plugsbadly worn orwrong type Leaking inletmanifold Carbon build-up incombustionchambers leadingto hot spots. Ignitiontimingincorrect Wrong gradeof fuel Carburettorbadly adjusted Idle speedset too high Anti-diesellingvalve not working

Running-on occurs when the fuel/air mixture in the cylinders ignites without a spark. This is known as the dieselling effect because it is caused by the fuel igniting spontaneously in the combustion chambers, which is what occurs (deliberately) in a diesel engine. There are several causes.

Check all the possibilities shown here, starting with the easiest ones.