I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.

That's my story and I am sticking to it.

--

*H. Allen Smith*
WACO - We are all here, because we are not all there.
"Todd W. Deckard" wrote in message
...
Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the
pressure difference in the carbureture venturi. So here goes:

The carb throat is a double venturi and a manometer between the opening
and the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the
fuel up (or properly the difference between the neck and ambient). Again
we use Bernoulli to describe the forces acting on a particle moving along
a streamline -- and this time it is properly incompressible.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity *
change_in_height = a constant

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2
+ (gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up
the gas becuase of the reduced pressure. So the gas drawn up stays about
the same, however the mass flow of air drops off with density so the
mixture richens.

Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!