Engine stumble, Any thoughts?

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 ??

--
Roy
N5804F Piper Archer

"I have had some bad landings but I have never missed the runway"


"Dave Butler" wrote in message
news:1133797784.923574@sj-nntpcache-3...
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.....

I'm a little hesitant to reply since I can't supply details or references,
but what the heck, this is usenet.

I've owned an Archer (carbureted O360) and have heard of other Cherokees
with this same symptom. I was told there is a transition in the carburetor
at about 1500 RPM, and that many of these engines will stumble across this
transtion. Maybe it's a transition from the idle jets to the main jets(?),
I don't know what I'm talking about.

Anyway, I spent a lot of time and money trying to diagnose this symptom in
the Archer. Two owners later, the airplane is still around at my home
airport, and I'm told it still has the stumble. It gets you when you're on
short final and reduced power, and decide you need a little more power to
get to the runway.

Dave

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
??

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

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

allowing the fuel mixture to collapse before the new lower
velocity swirl pattern establishes itself... It is a lean stumble in
my book..

Does anyone have a Cessna that does this? or might it be specific to
the Piper/Beech installations? I have never encountered it on my 172M
- and I lean a lot.

On 6 Dec 2005 04:22:14 -0800, "Denny" wrote:

snip for length, not content

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...

Am a flat-lander that hasn't done a lot of hot/high TO's & L's. 99% of
my pattern-to-approach-to-land has been full rich. When learning to
properly manage a TIO-540, lean mixture was typically used during
descent to help keep temps up/rate of change down, but any power
reduction was relatively gradual-and the mixture knobs went to the
panel entering the pattern.

Have time working on/flying in a bunch of carb-d Pipers, but the
Apache is one that I missed out on.

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..

I'd buy into that one. Still flat-ass amazes me that a tractor
carburetor works as well as it does. Have been around a bunch of
Archers that stumbled on power reduction going downhill. Again,
mixture ususally full rich, and what I would consider to be a
relatively gradual power reduction. Screwed around with a couple of
them with regard to playing with the mixture, engine relatively
hot/cool, messed with plugs, timing, etc. Were company airplanes not a
customer's, so really wasn't sticking it to anybody, just curious.

Regards;

TC