black smoke usually indicates unburned gasoline, the black is carbon. try to find info on adjusting the carburetor, should be something online about that. for future reference, blue smoke is oil, means your piston rings are going. In that event, you may be able to forestall a complete engine failure with the addition of STP or Motor Honey ( by Casite) to the crankcase of the mower. try to use in proportion to label's recommendation for a car.