Here's a pic of the green wires on the back of the fuel gauge that I want you to pull off the spade terminal. This is a pic I took of when I wired in a 10v Smiths regulated voltage supply which I found made the accuracy of my fuel gauge better. That's what I'm holding in my hand. Anyway, the fault is either on that green feed wire that goes from the ignition light, to the fuel gauge, to the tach, and on from there OR the fault is on the other side, which is the voltage regulator. Either way, it should be easy to trace.
