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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
Thank you Husqvarna man, and Phil, I will check both.

simon, April 2010
I assume the air filter is clean?

Husqvarna man, April 2010
Depending on the age, it's probably not adjustable. It sounds like you're describing a 'Classic' type engine with the carb on the tank. The diaphragm needs replacing.

phil_saunders(a)bigfoot.com

Phil Saunders, April 2010