On Sat, 16 Jun 2007 16:52:43 -0700, Peter C
wrote:

Mountfield petrol rotary mower with a Briggs Stratton engine.No
problem starting. When accelarating to cut the lawn it is mis firing
and black smoke emits from the exhaust.
New plug fitted. cleaned air filter.
Where do I go next please.to solve the problem?
Peter.

Black smoke indicates that the fuel/air mixture is too rich.
Carburation varies greatly across the Briggs & Stratton engine range
so it's difficult to generalise about what to do. You need to check
the service info for your particular engine to find out how to clean
jets/adjust mixture. If you get BLUE smoke, that's burning oil caused
by engine wear.