I have experienced the same symptoms on my first Model IV.. I would first check to see if there is any seepage at the fittings at the tank outlet. The outer bushing that actually mates to the tank can leak as it is brass to fiberglass interface. That bushing is actually a finger strainer to catch large particles before they enter a fuel filter. Also most early fuel hose will allow vapors to escape through the hose. New non permeable hose is on the market. Others will offer leads on that. However, since that area is also the low point on the wing root, any leakage of the tank itself would manifest it as fuel odor in that area.