PA-28-140D Fluctuating RPM indicator

I purchased a 1971 Cherokee 140 shortly after getting back into flying the
first of this year. It is a marvelous plane, relatively inexpensive to fly
and to maintain.

The other night while flying with my instructor he asked me if I typically
got this much vibration. I told him that it felt normal, however when I
looked at the GPS on the steering column it was vibrating so much I couldn't
read it. All indicators were normal, the engine sounded fine, the one
exception was the RPM indicator was moving back and forth a couple of
hundred RPM from the 2500 I had it set at.

I haven't had a chance to fly again and my mechanic is on vacation until
tomorrow.

Any thoughts or ideas?

Charles Graef

I would open up the cowlings and look over the engine very thoroughly for
any obvious problems (cylinder cracks, loose spark plugs, loose spark plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they all
appear serviceable. Be sure to torque to the correct specification when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure to
clean the 'cigarettes' at the spark plug end of the spark plug wires with
something like MEK to avoid arcing around the spark plug. Then I would do a
quick run-up check and, if the problem persisted, ground the plane until a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was actually
going up and down several hundred RPM. A bouncing RPM indication is usually
caused by a tach cable in need of replacement.

"att news group" wrote in message
...
I purchased a 1971 Cherokee 140 shortly after getting back into flying the
first of this year. It is a marvelous plane, relatively inexpensive to
fly
and to maintain.

The other night while flying with my instructor he asked me if I
typically
got this much vibration. I told him that it felt normal, however when I
looked at the GPS on the steering column it was vibrating so much I
couldn't
read it. All indicators were normal, the engine sounded fine, the one
exception was the RPM indicator was moving back and forth a couple of
hundred RPM from the 2500 I had it set at.

I haven't had a chance to fly again and my mechanic is on vacation until
tomorrow.

Any thoughts or ideas?

Charles Graef

"Mike Noel" wrote in message
...
I would open up the cowlings and look over the engine very thoroughly for
any obvious problems (cylinder cracks, loose spark plugs, loose spark plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they all
appear serviceable. Be sure to torque to the correct specification when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure to
clean the 'cigarettes' at the spark plug end of the spark plug wires with
something like MEK to avoid arcing around the spark plug. Then I would do
a
quick run-up check and, if the problem persisted, ground the plane until a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was
actually
going up and down several hundred RPM. A bouncing RPM indication is
usually
caused by a tach cable in need of replacement.


Bingo. If the engine sounded right, I'm sure it was fine. A 50 rpm swing is
noticable. A couple of hundred rpm swing would be attention grabbing in the
"Hit hte NEAREST button on the GPS" kind of way.

KB

On 5-Sep-2005, "att news group" wrote:

Any thoughts or ideas?


A fluctuating tach with no other indication of engine problems is probably
caused by a binding tach cable. Fairly easy and (by airplane standards)
inexpensive to replace. However, if there was an accompanying unusual
vibration, it could be any number of things. Did you try running on only
left and right mags? If the problem is anywhere in the ignition system
(mags, ignition harness, plugs, that will confirm it and tell you which side
has the problem.
--
-Elliott Drucker

If you actually getting more vibration than normal and a fluctuating
tach then there is a problem that needs to be looked at before further
flight.
Michelle

wrote:

On 5-Sep-2005, "att news group" wrote:



Any thoughts or ideas?




A fluctuating tach with no other indication of engine problems is probably
caused by a binding tach cable. Fairly easy and (by airplane standards)
inexpensive to replace. However, if there was an accompanying unusual
vibration, it could be any number of things. Did you try running on only
left and right mags? If the problem is anywhere in the ignition system
(mags, ignition harness, plugs, that will confirm it and tell you which side
has the problem.

I ran the plane again late this afternoon, after de-fouling the plugs by
leaning it out during run-up at 2300 RPM. It flew fine, I didn't notice any
vibration, nor were there any wild fluctuations on the tach, I flew it
about 20 minutes. I then did a couple of touch and goes at a nearby
airport. On the way back to my home base again I saw the RPM fluctuate and
I thought I detected a little roughness in the way the engine ran. There
certainly has been no corresponding fluctuation in engine noise with the
fluctuation in the Tachometer.

Thanks for everyone's suggestions, I am going to have my mechanic look at it
tomorrow. I'll let everyone know what he finds.

Charles Graef


"Kyle Boatright" wrote in message
...

"Mike Noel" wrote in message
...
I would open up the cowlings and look over the engine very thoroughly for
any obvious problems (cylinder cracks, loose spark plugs, loose spark
plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they all
appear serviceable. Be sure to torque to the correct specification when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure to
clean the 'cigarettes' at the spark plug end of the spark plug wires with
something like MEK to avoid arcing around the spark plug. Then I would
do a
quick run-up check and, if the problem persisted, ground the plane until
a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was
actually
going up and down several hundred RPM. A bouncing RPM indication is
usually
caused by a tach cable in need of replacement.


Bingo. If the engine sounded right, I'm sure it was fine. A 50 rpm swing
is noticable. A couple of hundred rpm swing would be attention grabbing in
the "Hit hte NEAREST button on the GPS" kind of way.

KB

Well the verdict came in today, the Tach cable needed to be lubricated.
Problem solved. Thanks for all the feed back.


Charles

"att news group" wrote in message
...
I ran the plane again late this afternoon, after de-fouling the plugs by
leaning it out during run-up at 2300 RPM. It flew fine, I didn't notice
any vibration, nor were there any wild fluctuations on the tach, I flew it
about 20 minutes. I then did a couple of touch and goes at a nearby
airport. On the way back to my home base again I saw the RPM fluctuate and
I thought I detected a little roughness in the way the engine ran. There
certainly has been no corresponding fluctuation in engine noise with the
fluctuation in the Tachometer.

Thanks for everyone's suggestions, I am going to have my mechanic look at
it tomorrow. I'll let everyone know what he finds.

Charles Graef


"Kyle Boatright" wrote in message
...

"Mike Noel" wrote in message
...
I would open up the cowlings and look over the engine very thoroughly for
any obvious problems (cylinder cracks, loose spark plugs, loose spark
plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they all
appear serviceable. Be sure to torque to the correct specification when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure to
clean the 'cigarettes' at the spark plug end of the spark plug wires
with
something like MEK to avoid arcing around the spark plug. Then I would
do a
quick run-up check and, if the problem persisted, ground the plane until
a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was
actually
going up and down several hundred RPM. A bouncing RPM indication is
usually
caused by a tach cable in need of replacement.


Bingo. If the engine sounded right, I'm sure it was fine. A 50 rpm swing
is noticable. A couple of hundred rpm swing would be attention grabbing
in the "Hit hte NEAREST button on the GPS" kind of way.

KB

FWIW, I too had bouncing tachometer syndrome and lubricated the cable. I
believe it was original and so about 32 years old. After the lubrication
the bouncing stopped but the cable broke in flight a few weeks later. The
replacement cost was in the $30 range.

You might also want to compare the RPM reading on your mechanical tach with
that of a hand-held optical tach pointed out the windscreen. Mine was
reading 80 RPM low, which explained why my fuel burn was higher than I
expected and the static RPM was lower than expected.

"att news group" wrote in message
...
Well the verdict came in today, the Tach cable needed to be lubricated.
Problem solved. Thanks for all the feed back.


Charles

"att news group" wrote in message
...
I ran the plane again late this afternoon, after de-fouling the plugs by
leaning it out during run-up at 2300 RPM. It flew fine, I didn't notice
any vibration, nor were there any wild fluctuations on the tach, I flew
it
about 20 minutes. I then did a couple of touch and goes at a nearby
airport. On the way back to my home base again I saw the RPM fluctuate
and
I thought I detected a little roughness in the way the engine ran. There
certainly has been no corresponding fluctuation in engine noise with the
fluctuation in the Tachometer.

Thanks for everyone's suggestions, I am going to have my mechanic look
at
it tomorrow. I'll let everyone know what he finds.

Charles Graef


"Kyle Boatright" wrote in message
...

"Mike Noel" wrote in message
...
I would open up the cowlings and look over the engine very thoroughly
for
any obvious problems (cylinder cracks, loose spark plugs, loose spark
plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they
all
appear serviceable. Be sure to torque to the correct specification
when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure
to
clean the 'cigarettes' at the spark plug end of the spark plug wires
with
something like MEK to avoid arcing around the spark plug. Then I
would
do a
quick run-up check and, if the problem persisted, ground the plane
until
a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was
actually
going up and down several hundred RPM. A bouncing RPM indication is
usually
caused by a tach cable in need of replacement.


Bingo. If the engine sounded right, I'm sure it was fine. A 50 rpm
swing
is noticable. A couple of hundred rpm swing would be attention grabbing
in the "Hit hte NEAREST button on the GPS" kind of way.

KB