Engine stumble, Thoughts?

I changed my mind assuming it is a lean stumble. A low float setting
and/or possibly an obstruction in the fuel delivery system would give a
low bowl level that could make the mixture go lean enough to stumble
from the main jet. In that case it would also be running lean at full
power - not a good situation.

This still doesn't fit every symptom. The cheapest way out is to get
an EGT - and soon..

OK..some ideas here are really good thoughts.

More info..

Have EGT, reading normal, can pull mix back 30% (with normal
EGT rise) to peak, and the mix operation appears normal.

This flight was in the coldest air experienced so far with our
"new" Warrior.(we bought it in Aug) DA may have been below SL..

One thing we will check is the fuel delivery. Not too suspect
yet as the fuel pressure is good with fuel pump off, plane is
hangared, and (so far) no water has ever been sumped. I suspect a lean
stumble.. mix was fill rich when it first happened. Tried both
tanks...same.

RPM at the stumble was about 2000 - 2200 every time, no
difficulty in the 1500 range. Smoothly closing the throt (rate of full
travel to full idle 3 seconds or more ) - no prob.... quickly closing
(not yanking) caused the stumble. Recovery to smooth was 1 second.The
rate of throt travel would be about 1.5 seconds to full idle to
produce the stumble.

Engine starts on 2nd blade with proper prime, smooth and pulls strong,
burns a liter oil every 12 hours.

Thanks to all who responded! We have several learned pinions to work
with.

Dave



On 5 Dec 2005 06:23:44 -0800, "nrp" wrote:

I changed my mind assuming it is a lean stumble. A low float setting
and/or possibly an obstruction in the fuel delivery system would give a
low bowl level that could make the mixture go lean enough to stumble
from the main jet. In that case it would also be running lean at full
power - not a good situation.

This still doesn't fit every symptom. The cheapest way out is to get
an EGT - and soon..

I have about 60 hours in a pa28-151 (160hp) that is a rental locally.
It does the same thing. The Mechanic as rebuilt the carb a couple times
and still can't figure out what is causing it. Only does it
occasionally and doesn't seem to require a fast reduction in power,
just a reduction in power. In fact it is more noticable when doing
smaller power reductions. It has been doing it for a couple hundred
hours now.

Would love to know what causes it as would the Mechanic.

Brian
CFIIG/ASEL

"Dave" wrote

.. quickly closing
(not yanking) caused the stumble. Recovery to smooth was 1 second.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Man goes to the doctor and says, "doc, it hurts every time I bend my arm
back and forth real quick, like this!"

Doc says, "Then, don't do that!"

My advise? If everything runs good, in every other way, keep an eye on
everything, and "don't do that!"
--
Jim in NC

Dave,

Your problem rings some bells for me since I have experienced such
problems with engines before. I have a pretty decent background. So,
I offer this.

The float level is probably okay. I normally do not see any
significant changes onse set. in fact, I have modified such settings
for efficiency. Setting the float lean, results in a lean situation
(bog) when accelerating or being placed under a heavy load. A rich
setting probably would not affect such a situation which you are
experiencing. A real real rich setting would be evident by observing
wet fuel within the venturi etc. This situation would also be noticed
by potent exhaust and would be evident at all times and RPM levels.
Thus, I do NOT think it is a float problem.

A vent hole plug could be a possibility but idle quality should show
this. Based on my experience, I would check for two probable
conditions. First, DIRT!!!!! I have found that dirt from fule tanks,
corrosion, etc. can develop into such hard to identify gremlins! It
has happened too many times! Ultimately, if let go, shall cause a
failure. Based on this, I would suggest several things. The carb,
really needs to be removed, inspected, and CLEANED! Then the float can
be verified (yes). All orificies need to be checked with light and /or
inserting a pin or fine wire. The light is good to see fine things
like strands of misc. stuck deep in passageways, etc. Then, once
cleaned, and checked, it needs to be assembled and adusted properly to
spcifications. While performing this function, INSURE that all housing
surfaces are FLAT and seal PROPERLY. Once this is complete, the fuel
filters should be replaced. You should also then, cut the old filters
open, and carefully look for an accumulation. Then, the air inlet and
filter assembly should be inspected and replaced if dirty. Once a new
filter or existing filter is re-installed, make DARN sure that the
intake inlet housing is totally sealed. If dirty air is being drawn
past the filter, it shall clog the air bleeds in the idle tube
passages. This could then, POSSIBLY cause your condition. If the carb
is in good condition, it would probably be more noticeable. Why you
ask. Well, as carbs age, performance at idle is usually compensated
with just incrasing the idle screw. That's okay, I guess. But on an
older carb with some wear, this puts some of the idling onto the main
circuit and covers up the problem on the idle circuit! Since the main
circuit can feed plenty of fuel, no noticeable problems can be felt!
But, if everything is in decent order, and the idle is truly adjusted
properly, a problem becomes more evident. I am not talking big
extremes here just small changes. Anyway, when everything is adjusted
properly, dirt, or some leakage, shows up much more easily ... like
idle air bleeds being clogged! If the carb is operating partly off the
mains, they are also operating of the main air bleeds also. These
bleeds would provide enough air, since they are designed for higher
RPM's. And, since the idle air bleeds are so much smaller, they are
easily plugged before the main bleeds would be.

The second are which needs addressing is a vaccuum leak. During rapid
throttle closing, the vaccuum spikes quickly which can expose and pull
outside air into the carb which results in a lean stumble! So, all
intake gaskets should be visually checked and even sprayed with say
WD-40 or carb spray and notice if RPM changes are evident. If they
are, then you found the problem!

Based on what you have described, you shall have to me quite meticulous
in every item which I mentioned here. I think that if you are, you
shall find the problem. Additionally, knowing the possibilities, when
you find the problem, it shall stick out like a sore thumb!

Finally, if the idle air bleeds are obstructed, a can of carb cleaner
would clear this up immediately. From there, it would be wise to pull
the top of the carb, and check for sediments and dirt. Based on what
you find, would determine if the carb had to be pulled. Normally, if
you catch a problem early enough (like you described), not too much
follow-up is needed other than removing the current problem.

Sometimes the simple things create the biggest hardships because they
are not always evident. But, you are doing the right thing! You noted
something wrong, and are investigating it, and you shall solve it, I'm
sure. Most people, would probably just dismiss it or worry about it
when it was convenient for them. But, left too long, can create a bad
situation!

Take Care, and I hope that this information helps!

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

Brian,

Please read what I just posted for Dave! I think you just answered the
question with your observation ... the mechanic has rebuilt the carb
several times!!!! I bet he is trying to adjust it to standard
specifications with a throttle shaft which bores are egg-shaped! This
causes leakage. It also creates proper adjustment problems. If you
set it to specifications, a stumble can result. If you set the idle
higher, then you are operating a little bit off the main feed system,
and then, the problem is covered up! But, then it is not as efficient,
and REALLY in fact, out of adjustment! Ideally, the carbs should
probably be replaced. but, as always, it comes down to money! YES!
Upping the idle is okay in most instances since there really is no down
point other than using a bit more fuel at idle. Old engines do not
function like new ones YES!

But, I think my response to Dave will also give you some good insight
in your inspections also. Dirt, buildups, wear, and leakage create
many problems and they come in many forms creating many weird gremlins!
ANY negligible movement of the throttle shaft affect the idle quality
and the functioning of the this fine adjusted system!

take Care!

Brian wrote:
I have about 60 hours in a pa28-151 (160hp) that is a rental locally.
It does the same thing. The Mechanic as rebuilt the carb a couple times
and still can't figure out what is causing it. Only does it
occasionally and doesn't seem to require a fast reduction in power,
just a reduction in power. In fact it is more noticable when doing
smaller power reductions. It has been doing it for a couple hundred
hours now.

Would love to know what causes it as would the Mechanic.

Brian
CFIIG/ASEL

Andre, thanks for this..

Your post is detailed, and encompasses most of the corrective
suggestions I have seen to date.

I must note that the stumble is at 2000-2200 rpm, just backing off of
cruise at 2350-2400...

The Idle is smooth and steady at 600-650. Throttle up is
smooth with no hesitation, plugs and exhaust are clean. No smoking.

This engine pulls strong, - comps all 75-77, ROC was 800 fpm
that morning , 2 on board (375 lbs tot) fuel at tabs,lots of winter
survival gear (ALWAYS PRESENT) and oat -5 C. Its starts/running on
the 2nd blade if prime is correct. Idle lean test is OK. Lean setting
changes in cruise do not seem to affect this stumble. (it first
ocoured at full rich at 3500 ASL) The stumble was repeated at lean r
of peak and l of peak. Throttle down slowly, NO SIGN of stumble,
operation is perfect.

This WAS the coldest day we have flown in so far (aircraft is new to
us)

Looks like the 1st checks we will do is look for an induction/carb air
leak that happens on a sudden increase in negative manifold
pressure. This could be worn bushings in the throttle valve, or some
other leak that has been "adjusted out" for other modes of oprn, but
not compensated for when the MP is suddenly high.. (?)

Thanks to this group, the Cherokee group, and a similar thread over in
rec.aviation.owning, we are gathering a wealth of information on where
to look.

The response was (delightfully) as expected....

No indication it is very serious, but it will be investigated, and any
progress will be posted.

Thanks all!

Dave


On 5 Dec 2005 17:34:13 -0800, wrote:

Dave,

Your problem rings some bells for me since I have experienced such
problems with engines before. I have a pretty decent background. So,
I offer this.

The float level is probably okay. I normally do not see any
significant changes onse set. in fact, I have modified such settings
for efficiency. Setting the float lean, results in a lean situation
(bog) when accelerating or being placed under a heavy load. A rich
setting probably would not affect such a situation which you are
experiencing. A real real rich setting would be evident by observing
wet fuel within the venturi etc. This situation would also be noticed
by potent exhaust and would be evident at all times and RPM levels.
Thus, I do NOT think it is a float problem.

A vent hole plug could be a possibility but idle quality should show
this. Based on my experience, I would check for two probable
conditions. First, DIRT!!!!! I have found that dirt from fule tanks,
corrosion, etc. can develop into such hard to identify gremlins! It
has happened too many times! Ultimately, if let go, shall cause a
failure. Based on this, I would suggest several things. The carb,
really needs to be removed, inspected, and CLEANED! Then the float can
be verified (yes). All orificies need to be checked with light and /or
inserting a pin or fine wire. The light is good to see fine things
like strands of misc. stuck deep in passageways, etc. Then, once
cleaned, and checked, it needs to be assembled and adusted properly to
spcifications. While performing this function, INSURE that all housing
surfaces are FLAT and seal PROPERLY. Once this is complete, the fuel
filters should be replaced. You should also then, cut the old filters
open, and carefully look for an accumulation. Then, the air inlet and
filter assembly should be inspected and replaced if dirty. Once a new
filter or existing filter is re-installed, make DARN sure that the
intake inlet housing is totally sealed. If dirty air is being drawn
past the filter, it shall clog the air bleeds in the idle tube
passages. This could then, POSSIBLY cause your condition. If the carb
is in good condition, it would probably be more noticeable. Why you
ask. Well, as carbs age, performance at idle is usually compensated
with just incrasing the idle screw. That's okay, I guess. But on an
older carb with some wear, this puts some of the idling onto the main
circuit and covers up the problem on the idle circuit! Since the main
circuit can feed plenty of fuel, no noticeable problems can be felt!
But, if everything is in decent order, and the idle is truly adjusted
properly, a problem becomes more evident. I am not talking big
extremes here just small changes. Anyway, when everything is adjusted
properly, dirt, or some leakage, shows up much more easily ... like
idle air bleeds being clogged! If the carb is operating partly off the
mains, they are also operating of the main air bleeds also. These
bleeds would provide enough air, since they are designed for higher
RPM's. And, since the idle air bleeds are so much smaller, they are
easily plugged before the main bleeds would be.

The second are which needs addressing is a vaccuum leak. During rapid
throttle closing, the vaccuum spikes quickly which can expose and pull
outside air into the carb which results in a lean stumble! So, all
intake gaskets should be visually checked and even sprayed with say
WD-40 or carb spray and notice if RPM changes are evident. If they
are, then you found the problem!

Based on what you have described, you shall have to me quite meticulous
in every item which I mentioned here. I think that if you are, you
shall find the problem. Additionally, knowing the possibilities, when
you find the problem, it shall stick out like a sore thumb!

Finally, if the idle air bleeds are obstructed, a can of carb cleaner
would clear this up immediately. From there, it would be wise to pull
the top of the carb, and check for sediments and dirt. Based on what
you find, would determine if the carb had to be pulled. Normally, if
you catch a problem early enough (like you described), not too much
follow-up is needed other than removing the current problem.

Sometimes the simple things create the biggest hardships because they
are not always evident. But, you are doing the right thing! You noted
something wrong, and are investigating it, and you shall solve it, I'm
sure. Most people, would probably just dismiss it or worry about it
when it was convenient for them. But, left too long, can create a bad
situation!

Take Care, and I hope that this information helps!

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