Is this while the bike is running, or not running?
If it happens while it is running, check needles/seats and float heights as previously mentioned.
if it happens while it is not running i would suspect the needles and seats, but your vacuum petcock might also be bad.
what year t500 did you end up getting? is there a picture thread yet?
actually after re-reading your thread, how much gas is coming out? is it constant or just a small amount after the tilt? starting to sound like the float level is just a little off.