They are not gravity fed from the bottom of the tank. Under the seat, at the top of the tank, there is a 90 deg Plastic fitting that screws into the tank top. The fitting has a plastic tube that goes from the fitting to the bottom of the tank. The fuel suction line is clamped to the outlet of this plastic fitting.
Also, keep in mind that a lot of these fuel tanks have a Vent Line, so blowing into the fuel tank inlet could be a waste of breath.
RD