Engine stumble, Any thoughts?

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

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.

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

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

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.

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

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 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

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

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