FuelSenderUnit
Fuel Tank Sender unit
The fuel tank sender unit in the X1/9 is prone to failure. These have a wire wound rheostat. When the thin wire breaks (due to corrosion or abrasion from the wiper) it will jump around as you describe. Mine would not read below half way. In any case, it is easy to remove. Once you have it out, connect it up and move the float and observe the fuel gage. You will quickly see the issue.
--Dom. 02:35, 9 January 2010 (UTC)
See photo below (courtesy of Bob Brown):
