running too rich check the air filter could also be defective injectors
Catalytic converters clog due to the carbon that the car produces. Usually, when the carbon has built up inside the cat., black smoke will come out of the tailpipe when you rev the engine. In extreme cases, black soot is visible on the ground next to the vehicle's tailpipe. Crystal Woll
It usually means too much fuel or not enough spark.
Running rich, depending on year of vehicle it's possibly a o2 sensor, fuel system problem, carb if so equipped. Time for you to do some diagnostics
You have worn valve seals and posibly bad valve guides.
soot
Purchase Soy candles that burn smoke free.....Enjoy!! No soot!!
check your converter let it run for ten mins or so and see if it gets red hot
Burning polystyrene from foam plastic packaging creates dense black soot particles. Burning tires does the same.
The organic compounds having high ratio of carbon burn with black smoke (SOOT) as Aromatic compounds for example Benzene, C6H6.
For steam from and, in emergencies, smoke to escape. CO2 is colourless and so when the smoke is white, it is steam. When it is black or grey, it is soot, carbon that has not combined with oxygen correctly.
engine tune just a major service then maybe a new exhaust system
The city bus emitted a steady stream of black soot from its tailpipe.