I am a lot more skeptical about venting a sump tank.  If the sump is vented and for any reason the mains don't gravity feed just right (it can and does happen - ask me how I know this :-) you can consume the fuel in the sump faster than it is gravity feeding from the main tank.  If the sump is vented, you will run it dry with lots and lots of fuel in the mains.  If you close the sump vent OTOH the sump level CAN'T go lower and the engine will "suck" fuel out of the mains to the sump (at least until one main empties and provides the sump with a "vent path" - you can ask me how I know this bit too :-).  I used to have my sump vent plumbed to the top of the cabin like Perry, and kept a dowel stuck in it to close it because I didn't want the sump to be able to empty before the mains.  One time I was driving along and smelled something and my wife looked over her shoulder and there was a fountain of fuel pouring out of the sump vent that had blown the dowel out.  She crawled into the back seat and stopped the leak with her fingers while I landed.  We drained a couple of gallons of gas out of the bilges. 

Now I have a schrader valve on my sump vent.  I take it off to allow the sump to fill all the way up and then cap it off again.  I know I'll NEVER fly with my sump vented, but like Perry I make sure the sump is full on preflight and if it isn't, vent it until it's full (and then cap it off).

Sump must be full and unvented to fly right ... Jim S.

Perry Mick wrote:

  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