Triumph Repair: Fuel Guage, voltage stabilizer, gauge wiring

Question
My fuel guage is always incorrect; either full or 3/4 or 1/2.  I'm not sure what to replace (guage, sending unit, voltage regulator.  The needle does move when the car is started.  Is there a test I can perform that will narrow the problem.  This is on a 76 Spitfire.

Answer
Hi Matt,
If the Temp gauge is not acting up too at the time the fuel gauge is reading wrong then the voltage stabilizer is not the problem. That leaves the gauge, wiring or sending unit.

Go to the sending unit and unplug the black wire (ground) and turn the key on, the needle should deflect in one direction all the way. Touch the black wire to the green w/ black wire at the sending unit and turn the key on again and the needle should deflect the opposite direction all the way. If it does this the gauge is most likely OK. Then jack the car up so the fuel will run away from the sending unit and remove the sending unit. Or drain the fuel tank. With the sending unit and wires as far from the tank as possible, reconnect the two wires and turn the key on and move the float up and down and note the needle positions. The float arm is common to stick due to being worn out or just rusty. Keep a fire extinguisher handy at all times and be careful not to cause a spark around the open tank.