On May 16, 6:40 am, john smith wrote:
In article ,
Cubdriver usenet AT danford DOT net wrote:

On 15 May 2007 14:35:47 -0700, "Robert M. Gary"
wrote:

In the C140, the Aeronca, and the J-3 the carbs (probably all
Stromburg), pulling the mixture all the way out at idle had no effect
at all on the engine.

I fly a J-3, and I've never seen one with a mixture control.

That's because the carburetor is hard to find now a days.
The ones that out there in the fly markets are high priced and are
pretty much junk. The people selling them are hoping to find a sucker
that only has the model number and doesn't know what to look for.
I am still looking for one to put on a 7AC Champ.

The Stromberg carb on that Cub will either have a mixture
control that's lockwired in the full rich position, or will be missing
the mixture mechanism and have a cover plate over it. My A-65 had the
cover plate, and since I was a machinist in a former life, I machined
the mixture control parts and put them in. It works fine, but I seldom
use it. The CHTs go up too much if I lean it.
The mixture control in that engine doesn't directy control
the fuel flow like the Marvel Schebler/Precision Aeromotive carb does,
and so it can't shut the fuel right off. The float bowl is vented
through a cavity in the top cover that has two other passages, one
leading to the dead airspace behind the venturi where the air pressure
is more or less ambient, and the other into the venturi itself.
Without the mixture parts installed, the venturi will draw a tiny bit
of air from the dead airspace, but not enough to drop the bowl
pressure. With a mixture control, leaning the engine reduces the
airflow from the dead airspace by gradually shutting it off, and the
venturi's much lower pressure begins to drop the atmospheric pressure
in the float bowl. Since fuel flow is dependent on the difference
between venturi pressure at the fuel nozzle and the bowl's vented
pressure, the flow decreases as the bowl's pressure comes closer to
the venturi's pressure. In other words, the venturi suction holds the
fuel back. At idle, there's not much airflow through the venturi and
it doesn't generate any suction, so the mixture has no effect on fuel
flow.
Turning a prop backwards during hand-propping reduces the
mixture ratio in the cylinders if the engine has been overprimed. Air
is drawn through the exhaust and will absorb the fuel, carrying it
back through the carb and out. The carb will often drip fuel during
such an event. Combustible mixtures range from 8:1 (rich) to 18:1
(lean), and overpriming will make the air/fuel mixture much richer
than 8:1 and the engine won't fire. When an electric starter is
available, we can pull the mixture, open the throttle and crank until
it leans out enough to catch, but when handpropping this could take
hours and cause a heart attack or something.

Dan