Try starting it with the gas cap off of the tank, just in case air cant get into the tank to alleviate the vacum caused by gas flowing out.
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Put about 2 feet of clear fuel line on the carb, do not connect it to the tank, but instead, leave it open (this could get sloppy if your not careful).
Carefully fill it with gas, and try to start the bike. you should see it go down.
These wont solve your problem, but should give you a better idea of where to look.