128L05 0116 F1 Briggs motor struggling.

[Bmwz3coupe]

I pickup a roadside Brute electric start walk behind mower. I went through carb throughly. Engines runs at 2700 rpm and sounds like it is struggling/rough not surging. The spark plug is sooty. I removed exhaust that made no difference. I put several ounces of Stabil Fast Fix in fuel as a long shot. I let engine run for 10-15 minutes but it was still struggling. I came back 5 minutes later and the motor was purring perfectly at a elevated rpm. So strong that when I engaged the drive lever the mower took off like a rocket. I was happy that the proble was fixed. Next morning I tried mower again and the struggling/roughness is back. What am I overlooking oh wisemen?

 

[Bertrrr]

You probably still have a dirty carb which comes from a dirty fuel system , tank, fuel lines, filter etc. if the carb was dirty at first and the tank and fuel supply system wasn't cleaned up too ,it got contaminated again I'd say