It sounds like it's something to do with your petcock if the gas won't stop flowing regardless of what positions its in (on/off/res).
But gas shouldn't flow out of your carb if your float is working correctly.
Try this first before you go hog wild re-building carbs and adjusting valves, but definitely check the petcock and clean out the gas tank.
"This same thing happened to me the other week.
I just finished loading my bike onto the trailer, and it just started pouring gas out of the over flow line.
I found a forum post where somebody said they took the drain screw at the bottom of the carb all the way out, let the carb drain, put it back in, and voila, no more over flow.
I decided to try that since it was 9pm and I was wanting to ride the next day.
I took the drain screw out, blew into the over flow line to blow out the gas from the carb, tapped the carb a few times with a wrench to make sure the float wasn't stuck, put the drain screw back in, and voila, no more over flow!
If it just started to do this on you randomly, I would try these first and see if it works.
I rode 4 hours the next day with zero issues, and its still working fine."