Glassair experienced fuel venting from the bottom of newly installed wing tip tanks during climb passing 3;000 FT. Flight returns to the departure airport and lands against the traffic with the aid of ATC.

Narrative

Fifteen minutes after departure and climbing to 13;000 FT I noticed fuel venting out the bottom of both wing tip tanks. The tanks are new and were checked for leaks in previous test flights but not after a prolonged climb and not with full wing tanks. [I] informed Center that I was making a 180 back to the airport and was venting fuel. They asked if I was declaring an emergency and I responded not at this time. When they handed me off to Approach they informed me that ATC was aware of my situation. Approach cleared me into their Class B and upon entering the airport's Class D airspace they told me to continue squawking assigned beacon code and contact Tower. I tried contacting Tower no fewer than 5 or 6 times with no reply due to radio congestion. I was descending at 200 KTS and closing on the airport fast. I have heard that a Glasair will loose all of it's fuel in less than 4 minutes if it loses a fuel cap (which was not the case) but did not know how much fuel I had lost or if I had enough to go around the pattern. I thought the venting would stop once the aircraft began to descend but it did not. There were 2 aircraft departing from Runway XXL and XXR (unseen by me at this point) and I was lined up for a straight in on YYL but unable to contact Tower. I was about to make a 360 and declare an emergency when Tower instructed the two aircraft to take evasive action and cleared me to land on YYL. I had both aircraft in sight at the time they turned away from me and landed without incident. The problem started with fuel venting which probably may have been detected with a longer test flight that included sustained climbs and full main fuel tanks. It would have helped to declare an emergency once I noticed that the venting was not stopping but I was distracted by the unfamiliar airspace; mountainous terrain; radio congestion; high volume of air traffic; and the fuel loss issue.