What causes an oil furnace to burn black smoke?

2 people have already told you what you need to do...call a pro when it comes to an oil burner. You have the basics of a Jet Engine running in your basement and while fuel oil is pretty safe if everything is right. There is a serious chance that something bad could happen if you work on it yourself without the proper training...its just too dangerous.What causes an oil furnace to burn black smoke?
To add my 2 cents worth, it takes very little time to completely clog a heat exchanger with soot if your fire is dirty enough to make black smoke come out of the chimney. Besides leading to a filthy clean-up, each 1/8 inch of soot build-up is equivalent to 1 inch of fiberglass insulation wrapped around the heat exchanger. In other words, less heat can get through the soot and into your exchange medium (warm air or hot water). The wind-up is that you're pushing dollar bills up the chimney as fast as you can, until the exchanger clogs and nothing goes through.What causes an oil furnace to burn black smoke?
Your furnace probably has too much oil and not enough air to burn properly. You need to get a heating technician to check it out. Might be something as easy as a good cleaning and adjusting will fix the problem.
could be many things or something as simple as a good cleaning. Like others have said find a reputable furnace tech to check it for you.
Incomplete combustion of the oil will do that.
bad.