Where do I begin...?

Look behind the gauge, maybe the positive wire touches the dash or some other metal part that is grounded.

 

More likely the sending unit is corroded and sticking. Try simple things first, as suggested by stratos. However, if the gauge and its associated wiring is okay, then you may be forced to remove the sending unit from the tank. Assuming this is a 246, the sending unit is on the passenger side tank. Simple to remove with 6 or 8 (can't remember) slotted screws. I would order a new rubber seal before diving in. It is likely dry and ready to be renewed. GT Car parts has them.

The sending unit can be accessed after taking off the stone guard of the right rear wheel. Before removing the sending unit, remove the 2 wires and clean the contacts, gently sand the male spades on top of the sending unit, gently tighten the female spades with a needle nose, plug them back in and check your gauge.

If you still have intermittent operation, then it is time to remove the sending unit. You will have to unscrew the slotted screws, (they are screwed into the aluminum tank, hence be careful when tightening that you do not strip the threads). Gently lift the sending unit out of the tank. I advise against using a cigarette lighter as a source of light. IMPORTANT: Before extracting, mark the top of the sending unit and the tank with a marker so that you can replace the sending unit in the same orientation. There is a long lever arm that will not function if replaced incorrectly. The tank has a slot built between the baffles to allow operation (rise and fall) of the float on its long lever arm.

Inspect the sending unit on the bench. If it is clean and without rust, then you are in luck. If you have an Ohm meter, you might consider checking the resistance as you move the lever arm from empty to full. You should see a smooth transition from few Ohms to many Ohms.

Check that there is no fuel in the float. If all is okay, replace and regroup.

I would not go this route until you have ruled out simpler causes.

Jim S.

 

Had a sender wire come loose during my rebuilds from hell. Got banged. Check that too. I got similar readings. On top of left gas tank. You can see with a flashlight from above and get to from the left rear wheel area. After wheel and shield are off.
John