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Causes of running on

Causes of running on Spark plugs badly worn or wrong type Leaking inlet manifold Carbon build-up in combustion chambers leading to hot spots. Ignition timing incorrect Wrong grade of fuel Carburettor badly adjusted Idle speed set too high Anti-dieselling valve 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.