On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
"Tman" wrote in message
Q: Why do I need to lean out my carb when I climb?

I'll take a stab at this one. Its a very good question.

A Stromberg carb does not require a mixture adjustment (at least below
8000'). It diverts low pressure air from the back of the venturi into the
fuel float bowl. In this way it is "self regulating" just as you describe.
The amount of
fuel drawn in is proportional to the air pressure.

Older classic airplanes will use this type of carberator system and thus
have no mixture.

The Stromberg carb's bowl is vented to a dead airspace behind
the venturi just like all the other carbs and has the same rich
problem with altitude that the others do. The Stromberg was built with
a mixture control valve cavity in the upper casting and many were left
empty and capped off to run full rich, or had the valve installed and
the lever wired to the full rich position. Most of the population, at
least years ago, lived near the coasts and flew puddlejumpers that
didn't fly very high. Fuel was cheap, too. So the makers didn't see
another control as having much value, expecially the back-suction
mixture type that the Stromberg uses and which will not act as an idle
cutoff for shutdown.
I have one of those old carbs on my airplane. I operate off a
strip that's around 3000' ASL. I machined the necessary parts for the
mixture control, they being very rare now, and installed them. It
works fine. It's a homebuilt and so such doings are permitted.
Air from behind the venturi where the air, being still, is at or
near ambient pressure, and this air passes into the cavity I
mentioned. There's a much smaller port in the cavity that leads to the
venturi itself and has considerably less pressure when the throttle is
open. Air is sucked out here. A third port into the cavity leads to
the bowl itelf. As long as the port from behind the venturi is wide
open, air can come in from behind the venturi and get sucked into the
venturi proper without exerting any negative pressure on the bowl.
When we lean, the mixture control simply starts choking off the air
supply from behind the venturi and allows the lower venturi pressure
to lower the bowl pressure, which reduces fuel flow through the jet
into the nozzle. Because the venturi's pressure drop is about zero
when at idle, it won't suck back on the bowl to act as an idle cutoff
at shutdown.

I believe more "conventional" systems use a mixture simply becuase the
logistics of balancing all the
jets is difficult and because slight misadjustments in the orifice that
tunes the ratio might result in
a catastrophically lean mixture. A carburetor can have four jet circuits
for idle, midrange, main and accelerator (I am quoting more Rochester
Quadrajet than Lycoming -- so aircraft mechanics please jump in).

The typical light-aircraft carb like the Marvel Schebler/
Precision Aeromotive/Tempest carb has one jet. The mixture control is
in the bottom of the float bowl and is a small valve that varies fuel
flow directly, from max to nothing at all.

Your mixture valve is in front of all of these and so restricts the fuel
thru all of them. If you have a higher performance
engine, or operate at a higher range of altitudes, I suspect you cannot
build a mechanical metering system that covers the range without regions of
overly rich or overly lean so we have the man in the loop.

Yup, you can, and it's done, too. It's just not cheap or
simple. There are various aneroid actuators that adjust fuel flow for
altitude, along with power valves that increase fuel flow for takeoff
and climb and other overriding devioces that prevent overly lean
conditions. Some pressure carbs (no float bowl) have these systems and
are similar in some ways to the fuel controllers used on fuel
injection systems. These types measure ambient air pressure, air
velocity through the carb, fuel supply pressure and fuel delivery
pressure and so forth and make the adjustments constantly.
This is diagram of a pressure carb, without the aneroid mixture
control. I can't find one on the 'net with it:
http://www.navioneer.org/riprelay/Th...bFlowChart.jpg


Dan