Black smoke from a car's exhaust is a sign that the engine is running too rich. The fuel is burning with incomplete combustion, and the black cloud includes the unburned carbon compounds. The NSW EPA says this is a sign the car is not properly maintained or poorly tuned. It is creating unnecessary air pollution and costing the driver more in fuel to operate.
If the vehicle has a dirty air filter, it does not allow sufficient oxygen to reach the engine's cylinders for complete combustion. Replacing the air filter on most cars is a quick and inexpensive fix that increases fuel economy. The operator's manual includes a recommended replacement schedule.
In vehicles with a carburettor, the black smoke is often an indicator of flooding or a butterfly choke stuck in the closed position. These problems are often due to the carburettor needing service due to varnish that builds up from the fuel as it evaporates.
The color of the smoke from the exhaust gives important clues to problems with the vehicle. Once they reach normal operating temperature, gasoline vehicles should not produce smoke. When the vehicle is cold, white smoke is usually water vapor from condensation and not a reason for concern. Blue smoke from a vehicle indicates excessive wear and severe mechanical problems.