You may want to try squirting some carburetor cleaner into the small hole on the underside of the fuel cap, to see if it is a dried fuel deposit (varnish) causing the problem.
Another word of caution with these fuel caps,if you overtighten the cap, it sounds like it's ratcheting, like the auto fuel caps from the same era as our bikes.
Don't tighten it up that tight, or one of these days, when you go to remove the cap to refuel the bike, the cap will just spin, and not come off.
Some feedback. I used the bike yesterday on a round trip of 50 km - no problem.
Used it today on a 50 km trip to Pretoria in bright sunshine - no problem.
Came back in pouring rain - no problem.
All that's happened is that I loosened the fuel cap 1/4 turn.
Oh, and I discovered how the cyl heads keep a patch of my jeans dry when it rains.
