Recently got this lawnmower. It cuts well, but after warm up it seems to be running too rich a mixture.. It occassionaly mis-fires and sounds like it should run better. The smell from the exhaust has a "tang" to it . There black smoke on start up , but this goes away after about 30 seconds.
There is no mixture adjustment on the carb - fixed jet.
How does the mixture adjustment work on this machine?
Is there a thermal "alve" that should open to allow more air?
Need the engine and date code for YOUR engine to help.
Is your choke partially stuck on all the time? Carb linkage messed up? Probably needs a "car-bu-trator" cleaning or replacement. You said you recently got the mower, sounds used possibly. Hope they didn't run E10 fuel in it. You can also slightly lower the carb float. This will lower the fuel in the bowl to lean it out a touch. Most Briggs engines run on the rich side.
There is no choke on this carb. When I got the machine - cleaned out the carb - was very clean to begin with.
There was no evidence of "E-10 Crud" - all passages in carb were OK.
The carb linkage is simple seems to work OK, from low idle to fast setting.
Still leaves the question of how the mixture gets changed as the engine warms up?
Take a picture of the valve cover or where ever your engine numbers are. Post it up here so we can see what you are working on. You have to look on the ENGINE, not the mower deck.
