"Rich S." wrote
I went back and re-read the entire thread and, you're right, I failed to
identify my airplane and to describe the fuel system configuration.

[Long, careful, and complete description deleted] Thank you. As I
suspected, you have a system that I always knew COULD suffer from
vapor lock. Insufficient head pressure for reliable gravity feed, and
a pump inline, mounted on the engine and thus sucking fuel.

There's nothing inherently wrong with that - every design is a
compromise - but there is an awful lot of FUD associated with the
autogas vapor lock issue. I know people who have had vapor lock
problems. I also know lots of people who use autogas in high wing
Cessnas and high wing Pipers, and NONE of those people have EVER
experienced vapor lock - even when running autogas in high
temperatures while crossing the Rockies, as I have. Clearly this is a
design issue.

Emeraude, powered with a Lycoming O-320-E3D and having a mechanical fuel
pump on the engine. There is a Bendix electric fuel pump mounted on the
engine side of the firewall, downstream of a gascolator.

I think these are the key points. Your pumps are in the engine
compartment, past the restrictions. Thus they are sucking fuel, not
pushing it, and are creating a point in your fuel system where the
pressure is lower than atmospheric ambient.

There is a fuel pressure gauge teed in at the carburetor.

Given the system, I think that's a fine idea. Generally, you do not
see fuel pressure gauges on gravity-fed carbureted systems. Generally
you DO see them on pump-fed systems. It's worth considering WHY this
is.

There are two 20 gallon fuel tanks, one located ahead of the instrument
panel and the other behind the seat. The nose tank has about 15" of head
above the carb inlet when the aircraft is level. There is a tank selector
valve mounted on the left kick panel. The rear tank is almost level with the
carb. The tank vents exit the bottom of the fuselage at 90° to the
slipstream and received no pressure "boost" from it. I have no idea how much
the restrictions in the fuel line (filters, check valves in the pumps,
fittings, etc.) affect the head pressure or fuel flow.

A fuel filter (if it's an actual filter and not a screen) will cost
you about about a foor or two of head pressure. Not sure about the
check valves. The fittings are basically irrelevant.

Just FYI - these things can be calculated. There are published data.
A sophmore-level textbook of fluid mechanics for chemical or
mechanical engineers will contain all this information.

On the day I experienced vapor lock and loss of power, the selector valve
was set on the nose tank and it was at least 2/3 full, adding another 10" to
the head distance. We were at 9,500' and the OAT was 85°. The electric pump
was turned off in cruise and the fuel pressure gauge had been reading just
above red line at ~2 lbs.

Just curious - is this normal for your installation? It seems very
low to me. In other words - it sounds like either your fuel filter
was clogged, or gas bubbles had already started to form in your fuel
system, causing pressure drop.

I turned on the electric pump and switched to the rear tank. Nothing.

I find this unsurprising - unless the vapor lock was between the
electric pump and the mechanical one, there would be no effect from
'sucking harder' when your problem was vaporization of the fuel in the
line. Since you have carefully insulated the fuel lines inside teh
engine compartment, there is no reason to believe the gas bubbles
would form there.

I admit that I had
forgotten to try the manual prime pump.

I doubt strongly that it would have helped.

I'm sorry to be so long-winded about this, but you asked for details.

And I'm glad you provided them. Especially details of the fuel
system.

Feel free to criticize my plane or my actions, but try to be kind.

Actually, I think all the actions you took were very reasonable. My
experience and ratings are similar to yours (though I racked up my
1500 hours in a little over a decade, and some of them in gliders and
light twins) and I doubt I would have done anything differently.

As for your plane - I will make this comment. The fuel system you
have is very similar to what one might expect to see on older
certified light aircraft, with one significant difference. An older
aircraft is most likely not going to have a real fuel filter.
Instead, it's likely to have a gascolator with a settling bowl and a
fuel screen - what's often referred to as an elephant catcher. Adding
a REAL fuel filter does provide additional protection for the engine,
but this protection is not free.

If there's one thing I've learned from my years as an engineer, it's
that everything is a tradeoff. That fuel filter protects your engine
from dirt and sediment, but it is a major source of pressure drop.
Worse, the more necessary that fuel filter is (meaning the more
contamination in the fuel) the more pressure drop it causes. I
suspect that if you had a simple wire screen, you would not have had a
problem. But that's 20/20 hindsight.

I'm just letting
you know what may happen if you persist in the belief that your airplane is
*immune* to vapor lock.

Actually, I no longer own that TriPacer. But I do believe that a
plane with a properly designed gravity fed fuel system is going to be
immune to vapor lock. I believe this based on the calculations
(posted elsewhere in this thread) and based on the field experience of
those who have been doing it.

I also believe that any plane with a suction-pump fuel system is at
risk for vapor lock. That doesn't mean we should not build such
planes (everything is a tradeoff!) but that we should be aware of the
issues.

To me, this is just one more issue where the design of the systems
makes a difference in what constitutes acceptable operation.

Trust me in this, though. You don't ever want to
hear the person you love screaming and sobbing in fear. Never.

I'll take your word for it.

Michael