Originally Posted by 
slug_burner
				 
			have looked at workshop manual, fuel and water temp gauge share the supply on a green wire.  fuel gauge has other side going off on a green black trace wire.
green/black goes back to fuel sender. 
the tank sender is a variable resistor one end and wiper connected to ground, the other end back to the gauge.
temp sender is depicted in the same manner, wire connecting sender and gauge is green/white close to the gauge then on the otherside of the connector it goes to green/blue.
the green/black from fuel gauge also goes to the low fuel level unit.
for some reason I think that the sender are 180 ohm units although I was not able to confirm it.
The gauges do have a damped response so once you swing your test pot to read full I expect the gauge to climb. 
Did you disconnect the sender when you used your test pot or did you throw it in parallel?
A little unfortunate/unlikely for both gauges to go at the same time!