Try cleaning or replacing the air cleaner. A clogged air clearner will make the engine run rich (like having the choke on). The rich mixture will cause the blackish smoke. The fileter may not even look very dirty, but the element can still be plugged and cause a problem similar to what you describe.
It could be that the mixture is a little on the rich side. Close (rotate clockwise) the needle valve on the carburetor about 1/8 of a turn.
Black smoke= running to rich(too much gas) try replacing your air filter or your carburetor might be out of adjustment, choke might be on White smoke=burning oil. Make sure there isn't too much oil in the crankcase, use the proper grade oil- sae 30w. wore piston rings or scored cylinder wall. could be a few more internal engine failures.
Because of the inconsistant revving and the puffs of black smoke, I would say that the air filter is clogged and/or the spark plug needs replacing. Else, the carburetor is set too rich or the choke is sticking.
One possibility is that the engine is running too rich. When you shut the engine off with no spark present the excess fuel present from the last couple cycles is flooding the engine. This would dissipate after time. Try the idling screw first ajust until the engine is about to cut out and then turn it about 1 turn back and see how it runs.You could also adjust the Pilot screw a tinybit toward the leaner side also. If this doesnt help then the float could be slightly sticking. Of course it could be some kind of screwy grounding problem. Hope this helps.
The fuel mixture is probably set to rich. Also, the wrong spark plug (with regard to heat range) could be in the engine. If you have an adjustable carburetor with mixture adjustment screws, try turning the screw (probably located on the bottom of the carburetor, in the middle of the bowl) inward 1/8 to 1/4 of a turn. This will lean the mixture. If you have a no-adjust carburetor, then there is not a lot you can do to change the mixture. Try using/replacing the spark plug with the correct spark plug as recommended by B&S. Good luck!
Sensors do not make an engine smoke, unless one is causing the engine to run rich.=Black smoke = Overly rich fuel/air mixture.==Blue smoke = Burning oil.==White smoke = Coolant entering the combustion chamber.==The O2 Sensor would be suspect if it is running rich.=
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There is no Oil lubricating your engine at a cold start up, so a rich mixture is used to try eliminating any scouring on your (internal) engine components.
Hell yes his rich, he makes 25 million dollar a movie!
rich makes about $100,000 per fight .