The petcock is a small valve that lets gas flow from the tank to the carb. It has a lever that can turn the gas "OFF" which stops the flow of gas. It also has settings for "ON" which is for normal riding, and "RES" which is reserve. When you run out of gas when the petcock is in the "ON" setting, you turn it to reserve to burn the excess gas in the very bottom of the tank to get you home. If you run out of gas while in "RES" then you are screwed. It should be located on the lefthand side just below the gas tank.
As for your tank cleaning...I don't know b/c I have never had to do it. (Thats the beauty of plastic tanks
)
[20:55] <waterpumper> putting a racing pipe on a Foreman is like putting a high dollar bikini on a 400 pound chick...just because it fits doesn't mean it looks cool