Just a wild guess, but it sounds like the petcock is stuck open. Might be in the Prime position or there is a problem with the vacuum valve on the back side of the petcock.
The vacuum valve is made up of a small diphram with a sping holding the valve closed. I would guess that there might be a piece of trash where the diphram seats or the spring might have broke.
Anyway that is what I would check at this point.
jnosal
Yellow 99 GS500E
Carefully read through the Please help, technical difficulties (http://www.gstwins.com/forum/viewtopic.php?t=9976) thread.
Thanks, i will look after that bad petcock