Why is the spark plug black on a Briggs and Stratton?

Answer 1

The fuel mixture is probably set to rich. Also, the wrong spark plug (with regard to heat range) could be in the engine. If you have an adjustable carburetor with mixture adjustment screws, try turning the screw (probably located on the bottom of the carburetor, in the middle of the bowl) inward 1/8 to 1/4 of a turn. This will lean the mixture. If you have a no-adjust carburetor, then there is not a lot you can do to change the mixture. Try using/replacing the spark plug with the correct spark plug as recommended by B&S. Good luck!