Proping Question

The magneto cannot generate a spark when turned slowly
backward, and it's not just because the impulse coupling doesn't work
backwards. The magnet could generate a flow in the primary winding,
but the points are closing rather than opening at the right time and
the arresting of the flow, a requirement for spark, isn't there.
Further, the distributor is geared to send a spark to a cylinder based
on a particular direction, and turning some reversible mags backward
can generate a spark but it happens when the distributor finger is in
the wrong place. Besides, the mag has to be turning at a good clip to
make any spark without the impulse coupling.
Worn-out vacuum pumps could indeed fail if turned
backward, but they don't belong on the airplane anyway. We use the
pumps with the wear inspection ports so that they never get to that
vulnerable stage. A good pump can be turned backward without fear of
breaking anything.
So we teach our students to turn the prop backward, but
that it must be done with great caution as a matter of course. Most of
them don't understand magnetos or engines or anything else and might
someday turn the thing in the wrong direction, and you have to make
such rules so that they learn that props can kill and so they will
keep their friends from fooling with it. We never lose a vacuum pump,
have never had an inadvertent firing. The most dangerous time is right
after the engine is shut down, when hot carbon in the cylinder head
could fire any vapors still present. We also check the mag grounding
at idle just before mixture cutoff.

Dan

On May 15, 8:20 am, wrote:
We also check the mag grounding
at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

"Robert M. Gary" wrote in message
ps.com...
On May 15, 8:20 am, wrote:
We also check the mag grounding
at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

This could be just a nomenclature issue. I was taught to call the lean
position of the mixture control "idle cut off" even though it really doesn't
cut anything off. However, it is too lean to keep the engine running at
1000 rpm.

I'm curious about what others think

Peter

On May 15, 2:08 pm, "Peter Dohm" wrote:
"Robert M. Gary" wrote in glegroups.com... On May 15, 8:20 am, wrote:
We also check the mag grounding
at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

This could be just a nomenclature issue. I was taught to call the lean
position of the mixture control "idle cut off" even though it really doesn't
cut anything off. However, it is too lean to keep the engine running at
1000 rpm.

I'm curious about what others think

Peter

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. The mixture control only affected the engine at
power.

-Robert

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. The mixture control only affected the engine at
power.


With the Marvel-Schebler carbs found on the average (post 1950s) light
singles, pulling the mixture all the way back will cut off fuel to the idle
circuit as well as the main jet.

John Galban=====N4BQ (PA28-180)

--
Message posted via AviationKB.com
http://www.aviationkb.com/Uwe/Forums...ation/200705/1

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.

Blue skies! -- Dan Ford

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.

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

"Cubdriver" usenet AT danford DOT net wrote in message
...
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.


In the C-120 I flew (IIRC it was a Stromberg) the mixture lever was safety
wired in place - there was no mixture control on the panel.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

On May 15, 10:21 am, "Robert M. Gary" wrote:
On May 15, 8:20 am, wrote:
We also check the mag grounding

at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

We were talking about turning the prop backward to reposition
it, not handpropping to start it. All our training airplanes have
starters and idle cutoff mixture controls. My own old Jodel with its
A-65 has to be handpropped, and has the old Stromberg with cruise
mixture control that has no effect on idle mixture. I shut the fuel
off and wait a bit for the bowl to run dry when I shut it down.

Dan