"Tman" wrote in message
...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. What do you see
wrong with the logic in this dialog?

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.

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).

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.

I am pretty sure of my answer but I'd invite any clarification.

Todd