<... path from the lagging tank gets hotter than the faster draining tank ...>
I hadn't thought of that, but I can't imagine how it could happen.  Sump sits across & behind the back seat about 8" from the firewall.  It might be the coolest part of the cabin (no possibility of direct sunlight).  Plumbing and vent routes are identical.  Problem is the same no matter where the sun is relative to the airplane.
Nope.  I can't think of what would make one line hotter or what heat would do ... Jim S.


Finn Lassen wrote:

The Cessna 152/172 have, what, 2 - 3 feet of gravity feed, you have only one.

Anyway, here's a question. Is there a possibility that the path from the lagging tank gets hotter than the faster draining tank? I'm beginning to think that even a minor temp rise in fuel path to pumps produces a much bigger effect than any of us would imagine. Just a theory not proven yet.

Finn

Jim Sower wrote:

This problem drove me NUTS for two years.  I tried everything.  For example, I checked the vents as best I could, leveled the airplane (checked the ball to ensure it was centered - it was), disconnected the fuel line from the engine side of the firewall and let it drain into a bucket.  I turned off one strake at a time (with manual maintenance valve) and timed how long it took for the other strake to gravity drain ten gal of fuel into the sump, out of the sump through the boost pump, through the filter and then through the firewall into the bucket(s).  Then I turned it off and did the same thing with the left tank.  They both drained 10 gal each  within 5% of the same time.  Then I reconnected all the plumbing, taped over the fuel caps so there's no possible way they could leak and went flying.  Right tank drained 20 gal while the left tank maybe 2 or 3.  Level balanced flight on a cross country.  All fuel lines, vent lines, etc. are as identical as I am able to make them.  To this day, I've not met anybody who can explain what happened.  I tried everything.  Back and forth on the Velocity list much longer than you've been inquiring after your vapor lock.  NOTHING.  I figured out a workaround - I installed electric shutoff valves in place of the manual maintenance valves between the strake and the sump.  Now, when one tank doesn't transfer, I turn off the tank that does and let the "reluctant" tank catch up.  The pump sucks it through the system just fine (the head pressure of 6" of fuel is about 0.16 psi - a problem that would inhibit that flow you can't even MEASURE.  Assymmetric transfer is common as dirt in the Velocity community (but not nearly as persistent as mine).  It was also a problem a long time with the Vari-EZ.