I had the same problem on my '91 GL; it was the "cross-vent" tube on top
of the tank, the fittings are plastic and eventually crack with age,
causing leaks and overflow when filling the tank.

Several of the well-known vendors on this list sell a "repair kit" for
this; it does require removing or at least partly lowering the tank.

Allan

On Tue, 15 Sep 2009 08:32 -0700, "Michael Kimball" <kimballmk@GMAIL.COM> wrote:

> Gas overflows on ground when filling.  Any similar experiences or
> fixes? I try to guess how  much to put in to avoid this.