fatdaddy's right. There's gotta be somethin. This is what mine looks like; http://motorbicycling.com/showthread.php?t=48562
I put a piece of rubber hose under the tank where the tube would be, and all I have to do is check the tightness once in a while. Works far better than I expected.
Gravity seems to be best with the majority of these set ups. There are carb options, as has been said.
Hope this helps.