Engine stumble, Any thoughts?

[Dave]

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

[nrp]

Not obvious yet. What kind of fuel?

Check the listed temps. The OAT will always be higher than the dew
point.

It is normal for a leaned engine running with carb heat ON to briefly
stumble (lean out even more) when carb heat is taken OFF. Doesn't
describe your case though.

[DavidM]

In my PA-28-161 (Warrior II), I occasionally get stumbles, both with my
old and new engine. Like you, I usually notice them in turbulence.

My guess (I don't have enough expertise to call it a 'theory') is that
the problem is actually a momentary change in the plane's angle of
attack due to turbulence. Since the Warrior has a fixed-pitch prop,
the RPM will drop when the AoA becomes higher. When the turbulence
goes the other way, you should get a power surge (as if the plane were
in a dive) -- I think I remember that as well, but I'll have to keep my
eyes open.

Does anyone have a better guess?


All the best,


David

[Michelle P]

accelerator pump mis-behaving?
Michelle

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
>
>

[Dave]

Whooops! DP was MINUS 10C... my bad.

100 Av gas

Nope.. symptom same, carb ht on or off..

TY!

Dave



On 4 Dec 2005 17:25:53 -0800, "nrp" >
wrote:

>Not obvious yet. What kind of fuel?
>
>Check the listed temps. The OAT will always be higher than the dew
>point.
>
>It is normal for a leaned engine running with carb heat ON to briefly
>stumble (lean out even more) when carb heat is taken OFF. Doesn't
>describe your case though.

[Denny]

This is common with O-320 carburetor engines when in the leaned
condition... Going full rich for three seconds before changing the
throttle normally smooths this out... A possible cause is worn shaft
bushings where you get air leakage past the butterfly shaft with
movement... Another possible cause is slop in the linkage causes the
butterfly to flop backwards as soon as you retard past the magic
balance point... In combination with the above two it may also be that
there is a butterfly opening below which the swirl pattern in the
intake manifold suddenly collapses before re-establishing a new swirl
pattern...
Or, it could be just bad karma...

denny

[[email protected]]

The mixture will lean somewhat while the throttle is
closing, and a weak ignition could cause the stumble as the mixture
becomes harder to ignite. Might check mags and plugs.

Dan

[Dave Butler]

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

[dlevy]

I have a musketeer with an O-320. I have a stumble also. I had the engine
overhauled (not because of the stumble) and it still does the same thing.
My opinion is that it is associated the intake manifold geometry. I am no
expert.

"Dave Butler" > wrote in message
news:1133797784.923574@sj-nntpcache-3...
><snip>
> 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]

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

[[email protected]]

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.

[Michelle P]

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 ??
>
>
>

[Roy Page]

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
>>??
>>
>>

[nrp]

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?

[Capt. Geoffrey Thorpe]

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?
>

[nrp]

>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.

[Rip]

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

[[email protected]]

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

[Capt. Geoffrey Thorpe]

"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.

[Denny]

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

[nrp]

> 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.

[[email protected]]

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