Ok, now I know you have an F650, although not what year.
If it's a 99 the vacuum hoses on the carburetors could be reversed. (from the factory - oops!)
The rear carb port hose goes to the charcoal canister, which I assume you have as you have a US address. The forward port hose goes to the airbox.
Otherwise, it could be just a kinked hose, so check very carefully that all the rubber hoses have nice smooth bends and no kinks. Did the problem start after you had just done some maintenance work? Also check that all the drain / vent hoses are clear. Disconnect and blow through them, and then make sure all hoses are securely attached.
I am still assuming you are just using 9 litres of fuel and there is still plenty of fuel left in the tank, it just doesn't get to it. If you are running out of gas and the tank is empty, that's another whole different problem.
For future problem solving, always try to give as much information as possible, e.g. what kind of bike it is, when the problem started, what steps you have taken so far to try and diagnose the problem, whether it only hapens at particular times etc.
Good luck, hopefully you have some ideas that will get you going. Let us know what the solution was for the next guy!
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