Pulled from the SoCal-Guzzi.com webboard:
Just finished repairing my gas cap for the second time. It wasn't too screwed up this time but I figured I'd get it before it gets me. I'm sure others have had the same problem on Cali caps, but this is what has happened to mine. The entire key assy. turns and you can't lock (or unlock) the cap anymore. What I've done is disassemble the lock assy. by removing the straight pin on the inside and epoxying the plastic pieces (2) at the cap (inside side). The first time I did this I thought I would make a temp fix. I used epoxy I had around (fuel resistant) but its lasted two years. This time I've used JB Weld (fuel proof), sanded and cleaned the inside (steel) of the cap with acetone. Cleaned the plastic with acetone (just wipe them clean). On the first plastic piece fillet a small bead at the base. On the second piece dab a little around the bottom where it lays against the cap. Mine was a little chewed where they join so with a toothpick I filled the gaps. Clean the excess of the ramps where the next piece will rotate. Let it all sit 24 hrs. before reassembly. You have to play with the thing a little putting it back together (it may be 180 deg. off) and put your key in the lock mech. when disassembling or assembling. I hope that makes sense and isn't like directions from a Harbor Freight whatsamacallit.
Larry K.