Engine stumble, Any thoughts?

Roy Page wrote:
We have the same problem with two Archers and a 180.
The problem existed before and after overhaul of both 0-360 engines [1 by
Lycoming & 1 by Signature]
Both rebuilds had overhauled carbs fitted.
The other Archer with 2100 SMOH has the same stumble at or near 1500 RPM.
A clever guy on the net will soon enlighten us as to the cause I am sure ??


Until I saw the other suggestions, which sound better, I was going to
suggest checking the primer lock. Whenever I saw something like this
on a Warrior with an O-320, this turned out to be the case.

Roy,
It may be a clever girl who enlightens you.
This has puzzled me. I am a Fuel Injection person (Jet engines, IO-540s
and the like).
He had an interesting thought based on his empirical data. The float is
the problem. It is riding too low in the bowl thereby allowing the level
of fuel to be a little too high. When the throttle is closed there is a
momentary lower pressure in the throat of the carb and sucking excess
fuel. Depending on how rich the charge is it may cause it ti stumble.
I was just looking into my Powerplant textbook. There is an Economizer
system that adjusts the mixture based on the throttle setting. Richer
for max power and leaner for cruise. If this is set up wrong it could
stumbler.

Either way the carb is most likely suspect.
Michelle

Roy Page wrote:

We have the same problem with two Archers and a 180.
The problem existed before and after overhaul of both 0-360 engines [1 by
Lycoming & 1 by Signature]
Both rebuilds had overhauled carbs fitted.
The other Archer with 2100 SMOH has the same stumble at or near 1500 RPM.
A clever guy on the net will soon enlighten us as to the cause I am sure ??

Michelle,
There we go .... a real clever gal.
Thanks Michelle, I am sure you are on the correct course with your
explanation.


--
Roy
N5804F Piper Archer

"I have had some bad landings but I have never missed the runway"
"Michelle P" wrote in message
ink.net...
Roy,
It may be a clever girl who enlightens you.
This has puzzled me. I am a Fuel Injection person (Jet engines, IO-540s
and the like).
He had an interesting thought based on his empirical data. The float is
the problem. It is riding too low in the bowl thereby allowing the level
of fuel to be a little too high. When the throttle is closed there is a
momentary lower pressure in the throat of the carb and sucking excess
fuel. Depending on how rich the charge is it may cause it ti stumble.
I was just looking into my Powerplant textbook. There is an Economizer
system that adjusts the mixture based on the throttle setting. Richer for
max power and leaner for cruise. If this is set up wrong it could
stumbler.

Either way the carb is most likely suspect.
Michelle

Roy Page wrote:

We have the same problem with two Archers and a 180.
The problem existed before and after overhaul of both 0-360 engines [1 by
Lycoming & 1 by Signature]
Both rebuilds had overhauled carbs fitted.
The other Archer with 2100 SMOH has the same stumble at or near 1500 RPM.
A clever guy on the net will soon enlighten us as to the cause I am sure
??

My 172M doesn't have any stumble for some reason. It is the rpm range
generally less than 1500 rpm though, where the carb has to evolve from
the idle jet system to the main jet. Carbs have a problem in this area
because this transistion simply involves a gradual change of the jet
system design, yet has to keep the mixture within a burnable range
(about a 9:1 to 18:1 air/fuel ratio) or there will be a stumble. I
don't know how carb designers (in the 30s I suppose) got it to work as
well as they do.

The main jet system is simple. The idle jet system is always a wierd
collection of ports etc.

Is it a lean stumble? or is it maybe a rich stumble?

When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off. My guess is a rich
stumble. But that's just a guess, eh?

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

"nrp" wrote in message
oups.com...
My 172M doesn't have any stumble for some reason. It is the rpm range
generally less than 1500 rpm though, where the carb has to evolve from
the idle jet system to the main jet. Carbs have a problem in this area
because this transistion simply involves a gradual change of the jet
system design, yet has to keep the mixture within a burnable range
(about a 9:1 to 18:1 air/fuel ratio) or there will be a stumble. I
don't know how carb designers (in the 30s I suppose) got it to work as
well as they do.

The main jet system is simple. The idle jet system is always a wierd
collection of ports etc.

Is it a lean stumble? or is it maybe a rich stumble?

When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off.

That's very true for a cold engine and especially with autofuel (which
I use in the 172M). In this case he was using 100LL in a warm engine,
so it wouldn't seem so likely.

I have never heard before of the kind of stumble given by the OP. It
could be a rich flash afterall.

Dave wrote:
Hi All!

OK, since the collective talent in this group greatly exceeds
mine and those around me...

Today the engine in our 151 (O 320-150hp)Warrior stumbled
during power reduction, and we were able to repeat the symptom.....

Here are details

OAT -5 C, 2350 rpm, kinda rough air, pulled back throttle to
decend after passing traffic..

I think we hit a bump which caused the throttle to be
retarded quicker than normal, engine stumbles for 1 second and
resumed operation at the new (lower ) throttle setting.

We were able to repeat this several times, carb heat on/off,
fuel pump on/off, leaned rich of peak/ full rich, left tank/right
tank, rt mag/left mag/both. - as long as we waited a couple of minutes
to try again. No evidence of carb icing, dew point was +10C

Moving the throttle immediately back to cruise, then sudden
retard, no problem. The stumble could be repeated by waiting 2 -3
minutes .

Recent maintenance, oil & filter change 1.5 hrs ago.

The stumble was like the mixture was momentairly too lean,
similar to the lean test during runup...

At 3500 ft, lean setting was at 60% of mix lever travel to
full rich..

Any thoughts from you engine talents out there?

Cheers!

Dave
Completely different engine, fuel set-up, and internals, but... the old
Continental E series engine will "stumble" if the throttle is reduced
too quickly from the crankshaft counterweights getting out of sync.
Lasts 2 to 3 seconds, generally going through 17 inches of manifold
pressure or thereabouts.

Rip

On Mon, 05 Dec 2005 19:34:20 GMT, Michelle P
wrote:

Roy,
It may be a clever girl who enlightens you.
This has puzzled me. I am a Fuel Injection person (Jet engines, IO-540s
and the like).
He had an interesting thought based on his empirical data. The float is
the problem. It is riding too low in the bowl thereby allowing the level
of fuel to be a little too high. When the throttle is closed there is a
momentary lower pressure in the throat of the carb and sucking excess
fuel. Depending on how rich the charge is it may cause it ti stumble.
I was just looking into my Powerplant textbook. There is an Economizer
system that adjusts the mixture based on the throttle setting. Richer
for max power and leaner for cruise. If this is set up wrong it could
stumbler.

One minor point, on a basic MS carb the "economizer" circuit is
essentially a calibrated vacuum leak into the fuel delivery stream. At
higher MAP/ambient differential it leaks more air, lower differential
it leaks less. If you have an economizer circuit, it is adjusted
according to the spec # of the carb, and can be closed off completely.
It is a "fixed" circuit, and does not vary mechanically with throttle
position.

FWIW, have seen a bunch of stumbling O-360's, but have never been in a
320 that had the problem.

Regards;

TC


snip

"nrp" wrote in message
oups.com...
When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off.

That's very true for a cold engine and especially with autofuel (which
I use in the 172M). In this case he was using 100LL in a warm engine,
so it wouldn't seem so likely.

I have never heard before of the kind of stumble given by the OP. It
could be a rich flash afterall.


I think you would find it to be true for warm engines also. There is a good
film of fuel on just about the whole surface of the intake manifold in just
about any carbureted engine.

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

Hey TC,
O-320 AIA in my Apache... They stumble when you retard the throttle on
downwind if they are leaned out at all...
The right engine was the worst, it would strangle and shake like a wet
puppy for about 1 second to 2 seconds ... I finally discovered that
carb was the wrong part number for the engine (It was a Cessna carb for
gawds sake) and put the correct carb on which greatly improved the
situation - a thousand dollars later...
Now, if you go full rich mixture for a second, before retarding the
throttle, they do not stumble... If you are leaned out and reduce
throttle at a normal rate they will burble for a second... If you have
been well trained, like my airplane has managed to make me, you
instinctively know to just how fast you can creep the throttle back
through that magic point so there is no stumble when leaned...

My own pet theory, based on a life time of building - and breaking -
engines is that there is a point where reducing angle of the throttle
plate (butterfly), too rapidly and therefore reducing the air velocity
rapidly, where the existing swirl pattern within the intake manifold
collapses, allowing the fuel mixture to collapse before the new lower
velocity swirl pattern establishes itself... It is a lean stumble in
my book..

denny