My Briggs and Stratton motor is running rich, how do I adjust the mixture?

When the mower is running, the engine sounds like it's hunting, reving and dropping. The choke doesn't seem to make a lot of difference. I have looked at the spring set up, under the air filter, but not sure how it's supposed to work, so I don't know if it is correct or not. The spark plug is black when I check it, which shows a rich mixture. Anyone know how to adjust it please? Many thanks.

Simon
April 2010