The engine is burning rich. Clean or replace the spark plug and air filter. If you still get black smoke, close (rotate clockwise) the carburetor needle valve 1/8 turn for a leaner mixture.
The three things that could cause black smoke which come to mind are:
1) You have a belt or pulley problem which is causing the belt to slip and burn.
2) You have an oil leak that is getting on to...
Common cause is burning oil.? no no no blue smoke is more from burning oil. black will come more from NOT burning all the gasoline in the fuel mix, and can be from loose or not secure vacuum. be sure...
More than likely, there is a stuck pulley that the belt is trying to spin. The black smoke is caused by friction between the belt and the pulley, heating up the belt. Check the pulley(s) for...