Actually, mine has been doing the same for a couple of years, now. No clues, other than it seems to originate from the tank area, rather than the carbs.
I have replaced the cap gasket and the sender gasket and haven't noticed any improvement. Thinking maybe a new cap, but they aren't available any more.
On my '85, the sending unit had been installed wrong by a previous owner, so I thought maybe you'd get lucky looking there.