Tony Cox wrote:

"Frank" wrote in message
...

snip

The explanation I got had to do with pressure (or lack of it) above
4500'. This problem is exacerbated by high humidity. It seems that the
plumbing allows a condition to develop whereby there is not enough
pressure from gravity to push the fuel through the plumbing without
forming "bubbles".
It has to do with the venting. Closing off one tank (ie selecting left or
right) solves this.

This admittedly vague explanation is only intended to point out that
pressure differences are the culprit. No one has ever shown me in great
detail just how it happens. Nonetheless, I'm a believer.


My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.


Right, lower fuel pressure means less oomph to get it through the
pipes....Seems consistent with the 'theory'. I don't know enough about it
to say whether it's right or wrong.


So I'm not really impressed at all by the explanation. Luckily,
engine power was restored when passing through 3500',
shortly (as it happens) after fiddling with the fuel selector as
part of the emergency checklist. Incidentally, that was the last
time I assumed that the POH for one plane was the same as
another.


I know the explanation wasn't very satisfying, I'd love to hear it explained
better too. But, as you have apparently also experienced, the phenomenon is
real. So there is something going on and pressure differences due to
venting/plumbing seems to explain it better than temperature related ones.

--
Frank....H