Sounds like a fuel pickup prob in the tank.

Check your internal tank fittings -- vent & pickup.

The vent tube should extend to the top of the tank & the fuel pickup obviously has the flexible hose & clunk weight.

Make sure that you aren't trying to feed from the vent tube. Also make sure that there are no pinholes or tears in the pickup hose & that the pickup line allows the clunk to swing freely without binding on the back end of the tank.