Vacuum and carb issues?

[thisisaggward]

hey there, i recently traded for a 1993 Seca 2 xj600s and when i test drove the motorcycle me and the other rider traded bikes for about a 60 mile ride. The bike rode fantastic and i was pleased so i made the trade. However 2 days later when i went to start the bike the air box filled with fuel and leaked out the drain on the bottom, a notable amount. My petcock is set to on and im not quite sure what the issue is.. the bike runs fine for about 15 seconds before fuel starts pouring out of the air box.
Any ideas are appreciated it.