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Common problem, caused by some parts in the combustion chamber becoming hot enough to light off small amounts of fuel without spark. It's very tough on the timing gears, and indicates at least a mild tuning problem.

Probable causes of that overheating, in no particular order:

1) Lean running mixture, not necessarily idle mixture. Classic symptom of SUs worn at the throttle shafts.

2) Too much spark advance for the fuel octane rating -- basically, prolonged mild pinging, not necessarily audible, causing heat build-up.

3) Not enough spark advance -- overall retarded timing regardless of fuel type.

With SU carbs, you can prevent the symptom by opening the throttles fully just as you switch off the ignition. That does not cure the mistuning, of course, but it does save embarrassment and timing gears.