Hugh, the below pic snagged from an earlier post in another thread by fellow Bolter Otto Skorzeny shows the installed orientation of the tank you mentioned and described by Waveski above.
Perhaps you could provide more details on the sending unit in question. Was it previously installed and accurate, or is this a new sender that you tested outside the tank before installing for the first time??? Is it possible the range of motion of the sender arm out side the tank is greater than inside the tank? I think you would need to double check that to be sure the arm has the same swing and isn't rubbing against the inside of the tank.
More details and perhaps a pic or two would help ID what may be going on.
