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Why is my B & S Sovereign 375 petrol lawn mower running erratically?

Hi, Can you please advise why my B & S Sovereign 375 petrol lawn mower is running erratically, lacking power (with lowish revs) and exhausting more smoke than normal. I've renewed the spark plug, cleaned the air filter and also added a petrol supplement designed for lawn mowers with no improvement.
Thanks
K James, July 2009
change the carb diaphram

Stuart, August 2009