When air/fuel ratio is too rich, more gas is being burned from a car's engine combustion chambers.
What causes the air fuel/ratio to be rich?
As you may not aware of, one of the cause is that the car air filter might be clogged, a clogged air filter will restrict the amount of air that reaches the engine combustion chamber. So, if black smoke from exhaust of car comes out the first thing to check is, if the car have a clogged air filter. Remove the car air filter and visually check if there is a lot of dirt on the filter. If dirt is present then this are more likely causes the air/fuel ratio to be rich. Just replace your car air filter to fix the problem.
If for instance, there is no presence of dirt on the air filter, another possible cause of the problem has something to do with your car fuel injection system, your car fuel injector sensor may have gone bad, The sensors measure various engine factors that use to calculate how much fuel is injected into your car engine cylinder, if the sensors gone badly the air/fuel ratio will be wrong. Sensors that will cause black smoke coming out from your car exhaust when gone badly are Mass airflow sensor, throttle position sensor and Engine coolant temperature sensor.
To identify what sensor have gone badly you will need to bring your car to a reputable auto repair shop and ask the mechanic to check your car with engine analyzer, just replace the sensor to fix the problem.
For car with carburetor possible cause of this kind of problem is a stuck choke plate, to find out if it is indeed a stuck choke plate problem, locate the carburetor, then remove the air filter unit to be able to look down the carburetors barrel, examine the location of the choke plate inside the carburetor barrel, if the choke plate is nearly close or close then you car is suffering from stuck choke problem and this causes the coming out of black exhaust smoke of car. To correct the problem apply choke spray to the choke linkage if this does not correct the problem you need a new choke.
If after the car was examined and find out that the choke plate is not stuck then the only remaining possible cause that makes your car to emit black smoke from exhaust of car is a vacuum leak, if this is the case then you'll need to bring your car to a reputable auto repair shop and ask the mechanic to test your car for a vacuum leak. Take this problem at once to avoid much bigger problem that may arise from having a black smoke from exhaust of car.