A float valve (or a few) might be NFG, or you might have a bit of dirt lodged in one. To flush dirt out, either ride with the petcock OFF until you run out of gas or drain the bowls manually (drain screws on bottom of bowls). Then open the petcock, the valves will be fully open and the fuel flow should wash away any clump of gunge.
Draining the bowls also gets any water/dirt/whatever out of them, but you waste some fuel. You can make a little trough out of tinfoil to take the fuel out to a jar or can: the fuel does not come out the overflow nipples (and tubing if you have that) it just leaks out around the screw once loosened. Letting the gas just pour onto the motor invites a serious fire, and the dripping gas will dissolve any tarmac under the bike anyway.
