On Tue, 20 Jan 2009 05:44:01 -0500, Tman
wrote:

Couple followups to the responses on this thread.

First, I'm not doubting that you have to lean at higher altitudes; that
is empirically most true. I'll try to restate the question below...
Stealth Pilot wrote:

"why do I lean my carb when I climb" is a bull**** question open to
much misinterpretation.

"why do I need to lean my carburettor at higher altitudes" is probably
a better wording of the question.

Sure, that's what I was asking in the original post.

that is simple. the air is less dense so the amount of fuel it needs
to achieve full combustion is reduced. the density of the fuel doesnt
decrease so you need less of it.

That I understand. It's also effectively a handwaving explanation to
the original question.

Here's the original question restated. Can somebody explain, why at
higher altitudes, the less dense air does not automatically meter less
fuel through a carb venturi, automatically making up for this
difference. A carb venturi operates taking advantage of the decrease in
pressure associated with the increase of velocity through the venturi --
a phenomenon which is dependent on the density of the air.


I think what actually happens in an aircraft carby is much cruder than
the elegant descriptions would have you believe.
in the marvel schebler ma3-spav there are a number of jets that all
need to be tweaked just so for the damn thing to work.

there is an accelerator jet which is pumped when the throttle goes in.
it also unpumps when the throttle gets pulled out but that just
refills it's barrel. the arm to this has three settings increasing or
decreasing the pump stroke and once you have this just sweet for your
engine dont change it. the outlet pipe needs to point just so into the
inner venturi ring.

there are a number of idle jets (actually just holes in the side) in
the wall of the carby that keep it running around idle when the
butterfly is almost closed. once this is adjusted for steady idle dont
change it.

then in the main jet comes in a whole number of sizes both in position
and size of the apertures that allow fuel to escape. there is also an
insert restricter in the base which is tweaked for each particular
engine type. dicking around with this to get a sweet running engine
can take ages.

as for the changes in velocity or density actually changing the fuel
flow well hmmmmm. if an engine can be run from sea level to about
6,000ft at full rich and still run and produce power, well that doesnt
seem like too much of a flow change with density.

jabiru use a bing altitude compensating carby. this has some sort of
altitude sensing compensation mechanism in addition to the usual
metering but that is a different animal.

the old style carbys have a lot of tinkering behind the steady smooth
metering in usual operation and have a simple adjustment that pilots
can use to keep the mixture in the runnable range at other times.

so my explanation is that if your surmise is correct I think it has
only about a tenth of the effect you think it does.
lots of tinkering and tweaking go behind making you think it does.

Stealth Pilot