Here's a link to everything you need to know about 90% of the fuel
level systems on small crafts.

http://ww2.tflx.com/pdf/marfue%7E1.pdf

(You'll need Acrobat Reader to view)

Armed with a couple of jumper wires alligator clips, an Volt/Ohm Meter,
and a potentiometer (variable resistor) you can diagnose just about all
of your gauges. The fuel gauge is in my opinion the easiest as you can
take the fuel sender out and test it by moving the float up and down
while hooked up to an ohm meter and see if it's sending the right
signal. Full or in the up position should read 30 ohms and Empty or
down position should read 240+/- ohms.

If that checks ok, while you're still at the fuel sending unit, using a
0-500 ohm variable resistor (I picked a 10 turn up at radio shack for
$8) hook it up to the ground and pink leads going to the sending unit
(Unhook them from the sending unit of course) and move it from 0-300+
ohms. This should move the gauge wherever you want it. (Don't forget
that the key will probably have to be in the on position to give power
to the gauge)

If the gauge isn't moving like it should, move up to the gauge itself.
Find a good ground for one jumper and the post that the sender feeds
into the gauge, usually a pink or red wire. Using the variable resistor
again, see if you can move the gauge by changing the resistance. If
not, you need to make sure the gauge has a good ground and that the
purple wire is feeding the gauge 12v.

Hope this helps!

Joe