easy way to see if your petcock is faulty... remove the fuel line with the petcock in the "on" position. it should not flow gas w/o vacuum applied to the backside of the petcock via that small vacuum hose.
your overflow problem could be from your float levels being off/misadjusted or a faulty fuel needle valve which controls fuel flow into the carbs.
did you redo the float levels when you did the rejetting?
edit... oops. forgot you may have a CA version bike. try TJ's suggestions first. 