Have you got the right tank ? - The injection tanks has a baffle/swirl pot that holds fuel at the pick up point (i.e @ pump up line) so when you go round corners all the fuel runs to one side of the tank but the baffle is still full. The carb version of the tank doesnt have the baffle.

You may have fitted the gauge sensor wrong - its easy to do. You could have fitted 90 degrees out ? That may account for the gauge reading wrong.

Hope this helps.