" jls" wrote in message
news
Anybody know the reason for this? The needle on the pilot's side is
stable, but the needle on the co-pilot side swings wildly, especially when
the tank is full. A float moves the needle around the dial. Why doesn't
it stay still and be good like its brother?

It probably is a bad sending unit, especially if it is a late model 172. You
can replace the sending units, but they just go bad again.