One final thought.....my aluminum sump tank in not vented. Seems we discussed this at length when I was building this thing, and since the overflow is pumped into the sump tank, the fuel might just be pumped right out of the vent. Take care. Paul Conner

Paul, I think this is your problem. I brought this up on this list once before. I nearly had the makings of a fatal accident a year ago when I temporarily had my sump tank unvented. I had been flying for over four years with a vented sump tank and never had a problem.
I flew to Puyallup WA for the airfaire early in the morning. The plane sat out in the sun all day, with temperatures rising. When I went out early afternoon to depart, I lifted the nose, hopped in, started the engine, taxied to active runway. Very fortunately there were several planes in the pattern and I had to wait for takeoff. Finally, I took the active, pushed in full throttle - and the engine died. This had never happened before. I got out and pushed it off the runway, somewhat stumped. Finally I remembered the rubber cap I had put on the sump tank vent. I popped it off and heard a "poof" sound, and then the sound of fuel flowing into the sump tank. My sump tank is mounted low, fed from the main tank by gravity. Apparently being parked nose down for several hours out in the sun allowed vapor lock to form in the lines between the main tank and the sump tank.  If those planes had not been in the pattern, I would have taken off from a relatively short field in a congested area and the engine would have quit right after takeoff with no place to land safely.

My sump tank vent is up at the top of the fuselage with the main tank vents, I never have problems with fuel being pumped out the vent. I now maintain that the sump tank must be vented - to do anything else may be fatal. I've brought this up before on this list and more recently on Canard Aviators.

Perry