How can I fix my push lawnmower Murray(Briggs and Stratton)20" 4.75hp Q?

It started putting out black smoke, then it started running uneven(it would run and lapse, run and lapse) then it stopped altogether. I checked the air filter, dirty as hell so cleaned it with K&N cleaner and oiled it with light coat of K&N oil. No dice, same story. Checked the spark plug: Corroded on the tip and the post and electrode were blackened. Post looked good though, not worn. The cable was corroded at the booth connector. Removed the sparkplug and cleaned it. Removed the sparkplug wire assembly which includes the "thing" that rubs against the other thing that rotates when you pull the rope(highly technical lingo). Cleaned the wire's booth connector. Separated it from the wire and cleaned the wire where it connects to the booth connector. Put it back together and added dyelectric grease. Added oil to crankcase. Still nothing. Sorry I cannot be more specific with the names of the parts but any help will be greatly appreciated.
My guess is that i need to replace the sparkplug and the sparkplug wire assembly and that this assembly needs to be gapped to the rotor thingy. Oh... by the way... what is the gap for the sparkplug? I have tried .014,.015 and .016 and no dice. (I dropped the sparkplug and it landed on the electrode, so I had to regap).