When an engine's fuel-air mixture is improperly mixed, with too much fuel being injected into the cylinders, it is said to be running rich. This causes black smoke in the exhaust, and sometimes adverse performance, not to mention a drop in fuel economy. Also there's a distinctive smell to the exhaust of an engine that is running rich, basically it smells like unburnt gasoline.

The reason this happens sometimes is because the fuel-air mixture ratio is not static to begin with; it gets richer under acceleration when compression levels are higher and more fuel is required, and also a richer mix is sometimes required for some cars where preignition is a danger, when temperatures inside the cylinders are hot enough to cause thinner mixtures to ignite prematurely, causing the piston to want to reverse direction which can really damage things. So for these reasons, the mix fluctuates, and like any other fine tuning of a car, sometimes it gets a little off.