What are the likely causes of a build up of carbon deposits on the jet of an oil
fired boiler?

The boiler was alternating between running roughly (nearly cutting out) and
running smoothly. When I looked at the jet there was a large build up of carbon
deposits. The boiler now runs smoothly with a new jet. As I have not had this
problem before I would like to know whether there may be an underlying cause
which needs attention. Is the carbon the cause or the result of the rough
running? - or even both.

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Michael Chare