85. A burner producing black smoke in an automatic auxiliary boiler, would be caused by a/an __________.

Black smoke indicates incomplete combustion, typically caused by insufficient primary air. Primary air mixes with fuel at the burner, and a deficient setting results in a rich mixture and soot production. Incorrect electrode settings, defective solenoid valves, and grounded high-tension leads cause ignition or fuel delivery issues, not sustained black smoke from a burning flame.