Beginning with the 1965 model year MGB and the 1967 model year MG Midget, power for the fuel gauge and coolant temperature gauge (on some cars) is controlled by a voltage stabilizer. If your instruments are acting up, particularly reading low or dead, this device may be the culprit.

This is a funny device that operates something like a turn signal flasher unit to switch power on and off in periodic square wave fashion. It varies from full system voltage to zero volts, and then back on again about once per second. The "on" time for each cycle is controlled to achieve a output averaging about 10 volts over time (but never actually 10 volts, just switching between full voltage or zero). This constant switching on/of confuses a digital volt meter, but it is clearly see on the swinging needle of an analog volt meter.

The voltage stabilizer may be mounted on the back of the dashboard, possibly on the back of the speedometer, or maybe on the firewall. It is to some extent position sensitive and may be marked for which side should be up. It also has to be grounded on the mounting panel to work, so if the mounting screws are loose it won't work. Also check and wiggle all wire terminals to be sure they have a good conection.

The fuel gauge has a small heater element which warms a bi-metalic strip. The bi-metalic strip bends with the temperature change to drive the needle on the gauge. This makes for very slow resonse and slow motion of the gauge, so you don't see the effect of the input power switching on/off, or the effect of varrying resistance of the sender unit from fuel sloshing in the tank.

The fuel gauge takes power from the voltage stabilizer. The second terminal on the back of the gauge is connected to the sender unit in the fuel tank. This varies resistance with fuel level, something like 320 ohms when empty and 90 ohms when full (but this range is different for different applications). If you short the fuel gauge signal terminal to ground it should peg the gauge on the "F" end of the scale.

The fuel tank needs to be grounded to complete the circuit. You can use an ohm meter to check resistance between the tank and the body of the car. It should be very near zero ohms If in doubt, connect a grounding wire from the tank to the chassis, and see if that cures the gauge reading.