If you didnt have fuel stabilizer in it, theres a good chance the floats are gunked up and sticking. Other possibilites are crap coming in from the tank (doubtful, if the tank looks good inside), Needle valves that arent sealing properly because of wear, Bad needle valve seat gaskets/o-rings, or possibly a bad petcock diaphragm thats allowing fuel to flow down the vaccum lines and into the float bowls. That should at least give you something to look for.
This is something you want to fix, pronto. Ive heard several horror stories about people losing their bikes to fire because a carb dumped gas on a hot pipe. Ive seen it happen on mine. Its a little unnerving to say the least. :shock: