Black smoking means that excessive amounts of fuel are being delivered to the cylinder for combustion. Since there is not enough air available to burn all of the fuel, some comes out as carbon particles, which are naturally black. Blacksmoking is usually accompanied by a strong fuel smell, as much of the fuel is simply exhausted in unburnt form. The engine will run poorly in this state, with reduced power and very poor economy. Your air/fuel mixture is too rich. You may have a clogged air filter. If the air filter is not clogged there may be a problem with a sensor controlling the vehicles fuel injection system. If your vehicle has a carburetor then the choke plate may be stuck. Another possibility is a vacuum leak.