M Series

Does your fuel gauge read ¼ full but you run out of petrol? If so, you may like to try this modification to rectify the fuel sender wire length.

The reason for the incorrect reading is that the float never reaches its lowest position (to the stop), as it stops on the bottom of the tank first. The idea is to shorten the float wire so that the wire hits the stop (on the sender unit body) just before the tank is empty. Bend it by making a few "zig zag" bends as shown in the photo. That way, you will get an accurate reading on the gauge. You may ask why not just bend the wire up slightly? Well, that is ok for the empty reading, but when the tank is full you will only see the gauge read ¾! The wire has to be shortened in length to make the float travel in a shorter arc - therefore reducing the travel (of the float) between the minimum and maximum stops. Simple really!
You could shorten the wire & make a new circular shape to hold the float, but you will have no room for error, so it's easier to bend it as shown so you can alter the length if necessary.
Actual removal of the sender unit is simple. Disconnect the battery (sparks are a bad idea!), take off the fuel tank cover, and remove the sender's locking ring in an anti-clockwise direction. Be careful not to damage the ring or the fuel tank, and no slipping as a spark may result! I tend to use a hard plastic ice scraper and a mallet as no sparks can occur, and less chance of damage. Under the fuel sender is a rubber seal - always replace this if at all damaged as when the tank is full you can get seepage. Never use a cork seal as they allow gas to escape and your car/clothes will smell of fuel.
When refitting the sender, make sure the locking ring is central (it can go to one side and not seat properly). Also I have heard of the solder breaking away on one owners fuel tank when tightening, so be prepared for this (may be poor factory soldering on the odd tank).

If you have run out of fuel due to this problem then I recommend you try this - it worked for me and I no longer run out of fuel.