4.5 hp briggs and stratton running very poorly.?

I have a one year old weedeater lawn mower with a 4.5hp briggs and stratton engine. The mower ran flawlessly all last year, and most of this year, till about 2 months ago when the neighbor borrowed it and fueled it up. The mower sputtered and ran horrible after that, so I emptied the tank which had quite a bit of water in it. Refilled and stared the mower and it ran poorly again, so I emptied the tank, removed the plug which was severly carbon fouled, and cleaned the air filter while I was at it since it was dirty. I put it all back together and it still spit and putted around, and revved like it couldn't control the rpms. I took it apart again, cleaned the carb, which is nothing but a cheap piece of plactic, put fresh gas in again, removed the plug which was fouled again and replaced it, cleaned the filter again just to be sure, and checked the two springs for the governor to make sure they weren't missing or something wrong, which nothing was. I put it all back together, and it doesn't seem to spit anymore but it revs uncontrolably now, I can watch the governor move back and forth as it revs then backs down. The mower could be started by a 4 year old I might add, one gentle pull and it starts every time. What is up with this thing?