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My "great buy" LA105 starts, runs rough, then smooths out. After a minute or so it begins to surge and belches black smoke. Plug is fouled with soot. Can you point me in a direction to start, please.
Thanks, John

It's a Briggs & Stratton 19.5 hp engine family is 9Bsxs 5002VP. Air filter is clean, I will change fuel filter next.

If the carburetor has a plastic float valve replace it with a brass .

I wouldn't be surprised if the plastic choke shaft in the carb is warped and is sticking in the choke closed position and not opening when returned to the non-choke position.