Probably the guage wire has come loose.  I *think* they show empty when
the wire is grounded, and full when it has high resistance.  The sender
is pretty easy to get to on a Syncro -- you can see it inboard of the
left rear wheel.  There is a two-wire connector that should be visible
in front of the shock tower.

If you unplug the wire, and jumper the two connectors, the gas needle should
move.  If it does not, then the wire is broken.  If it does, the either the
sender or the connection is bad.  Mine had well-corroded contacts, since the
rear wheel throws lots of crap into this area...

malcolm