1009 02 06 - I filled the aircraft this morning. At the same time I again filled the area behind the passenger with about a gallon of avgas. BUT, this time I was able to determine the source of the problem. It was NOT the vent system. When I was planning to install the aux fuel tank, I put in all the fittings. Later, my wife explained that if I put the aux tank in, she wasn't flying. She liked the fact that the Europa would have to stop every three hours, she didn't want the ability to fly for 5+ hours. Also, any reduction in baggage payload was TOTALLY unacceptable. So, no aux tank. I pretty much stopped installing. Dumb move. I had installed the vent hose and self-closing fitting for the aux tank, but I didn't put the hose and self-closing fitting on the fuel inlet. The little barb fitting was just sticking out between the main fuel inlet and the upper vent fitting. (BTW, remember, 8 folks, all homebuilders, went over this aircraft with fine-toothed combs and all missed this one) When I'd run fuel in too fast one of the ways to relieve pressure was to blow fuel out that little barbed fitting and into the back well area. Also, when the tank was filled, any excess fuel burbled out of the little barbed fitting into this area. By installing the tube with the self closing fitting the fuel spill issue should be corrected. The aircraft is presently open and allowing any residual fuel to evaporate.