Exact same problem here (loose stopper) on my first Avistar flight for the year. I first noticed a problem when the aircraft pitched up sharply when I increased power in flight (fuel sloshing back in the cabin, I later concluded). I decided I had better bring it in to see if the wing bolts (recent mod) were loose or something, and it deadsticked during approach. Landed fine.

Everything seemed okay in terms of controls, wing, etc, so I started to fuel her up again.. I got suspicious when after a 3-minute flight it took the normal length of time to fill the tank again, then gasped in horror when I saw fuel dripping down through the landing-gear mount point. No smoking servos, but it was a big mess in there. Given what fuel does to balsa/glue the plane's days are probably numbered... You cant fuelproof the ENTIRE inside of the plane, and when that much fuel is washing around it gets everywhere!

-Greg