Correct answer
Incomplete combustion from insufficient air or excessive fuel.
It can indicate incomplete combustion producing soot because there is too much fuel for the available air. Possible causes include restricted air intake, for example a clogged filter, or incorrect fuel delivery. A shortage of air does not mean the system should compensate by injecting more fuel. Smoke colour alone does not identify a particular fault; the system needs checking.
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That there is necessarily too much air in the mixture.
Incomplete combustion from insufficient air or excessive fuel.
That only oil is burning and the fuel is burning perfectly.
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