Put a fuel shutoff in the gas line. Gas is leaking as the needle valve is not sealing. Probably small dirt/grass particle stuck on needle. Don't fight it just put a fuel shut off - turn off after use. Mine did the same thing.

Add an inline fuel filter when you add the shutoff.

And check that the exhaust ports are clear, clean.

Walt Conner

This will undoubtedly be met with some skepticism. With the engine turned off, pick up the rear of the mower and let it drop from about 6" once or twice. Most likely the float is a little stuck, and this usually frees them. Definitely install the cut-off as the others said. Add 1 oz./gal of Marvel Mystery Oil to your mix can in addition to the usual oil. If you use the choke to start it plus the primer, just be sure to pull the choke off quickly after it starts. The 'black liquid' results from excess liquid fuel in the transfer pipe and muffler washing carbon off and blowing it out the exhaust. Anytime you see this, do the drop. Also be alert for a fuel leak when you put the mower away - they have been known to empty the tank just sitting there. The only good in all this is that 2-cycles love fuel! It is amazing how they can start while being flooded like that.

The Dura Force models seem to leak at the float valve more than other 2 stroke Lawnboy models. Run some Seafoam at a high dose (2oz per gallon) for a couple of tanks. Then, like saxman said, run MMO in your fuel all the time.