Tony Cox wrote:

"Frank" wrote in message
...
Tony Cox wrote:

Some models of 172 *require* you to select a single tank
above 5000', due (supposedly) to vapor lock problems.


No 'supposedly' about it. Ours locked at 7500' on a humid day because one
of
my partners didn't believe it could happen and didn't follow the
procedure.
Fortunately an airport was within glide range. Strangely enough, by the
time he landed the engine was running fine again....
--
Frank....H

Its happened to me too. I said 'supposedly' because I've not
heard a plausible explanation as to why selecting one
particular tank rather than both should affect the chance of
vapor lock. Is the fuel selector close to a heat source?


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.

--
Frank....H