Good luck with it Ned.

Don't forget you are working with a combination of petrol and electrickery...

If we are talking MGB based here, then it is likely to require a ballast resistor in the line, which is usually screwed to the back of the speedo (or is it tacho)! The purpose of this unit is to damp out the fluctuations caused by the fuel moving around in the tank when moving and acting upon the sender unit.

As advised, check connections and earths and consult the MGB wiring diagram.