I had a similar problem with my lawnmower (also 3.5hp B&S), which cleared itself up after using it for half an hour. What had happened was that the oil from the sump had gone into the carburettor or air filter, and was being fed in with the petrol. I'm betting you turned the machine over, or onto its side, to look at the damage.
In my case, I'd also put a bit too much oil into the sump, which didn't help.
Alvery
July 2008
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