Soot on coils/burner assembly

A burner that is improperly  adjusted and has been blowing black smoke will build up soot on the coils and insulation.  One that is not getting good ignition will get excessive soot to the point it will start smoking.  

Even if you have cleared up the initial problem you may continue to have smoke problems or even a fire inside the burner.

If it's only a small amount of soot on the coil you may find it clears itself up after you have fixed the initial problem.  Larger amounts of soot will not only diminish the heat transfer but will continue to smoke.  There are products on the market such as Red Devil that you add to your fuel to remove the soot.  For maximum efficiency of your burner it is suggested you add the product on a regular basis.  But, there is no substitute for proper burner maintenance.

A burner that has been running without enough air or improper igniter setting can experience oil soaked insulation.  If this is the case you may see heavy black smoke and/or a fire inside the chamber.  In this case the insulation needs to be replaced as no cleaning will get the oil out of the insulation.  If you let it burn, eventually the smoke will go away but so will the insulation.  Also, your neighbors or authorities might have something to say about it!