Bob,
The 78 Royale utilizes a standard 0-90ohm balancing coil fuel gauge, There are three terminals on the back of the round 52mm (2 1/4") gauge. The one marked (+) is 12v power from the ignition switch source. The one marked (-) goes to the sending unit in the tank. The third one, usually located between and below the other two is a ground. (not usually marked) The ground at the gauge is essential if you want the gauge to work. Without it, the needle will deflect either full or empty (depending on how it is wired), with no in between deflection indicating fuel level. Check and make sure your gauge is wired properly.