Pawnee problems

On Apr 29, 11:13*am, "bds" wrote:
Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. *We are having intermittent problems with power surges that
can be significant and are sometimes even noticeable from the ground. *The
problem will occur one day on one tow, and then sometimes disappear for
several weekends. *One tow pilot seems to think that the problem seems to
appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for obstructions,
and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. *We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might have
started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses occur.
I have never experienced one of the major losses myself, but I have had a
few of the 100 rpm drops which cleared up after about 15 seconds or so.
When it happened to me I did notice a very slight engine roughness which
does sound like it could be a stuck valve. *It also makes sense that if it
is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? *Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need our
Pawnee back on the flight line. *We are close to doing the top overhaul as a
next step.

Thanks,
Bruce

wing tanks or header tank? has there been any consistency in fuel
level during these events? ive had engine problems in airplanes with
header tanks at extreme nose up attitudes and low fuel levels. your
"negative G" towpilot could be on to something. is there an engine
driven fuel pump on the pawnee, even with a header tank? have you
checked to make sure it is working?

ive had a valve stick (on a cessna 150) and it was VERY noticeable,
not "very slight" roughness.

On Apr 29, 10:13*am, "bds" wrote:
Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. *We are having intermittent problems with power surges that
can be significant and are sometimes even noticeable from the ground. *The
problem will occur one day on one tow, and then sometimes disappear for
several weekends. *One tow pilot seems to think that the problem seems to
appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for obstructions,
and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. *We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might have
started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses occur.
I have never experienced one of the major losses myself, but I have had a
few of the 100 rpm drops which cleared up after about 15 seconds or so.
When it happened to me I did notice a very slight engine roughness which
does sound like it could be a stuck valve. *It also makes sense that if it
is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? *Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need our
Pawnee back on the flight line. *We are close to doing the top overhaul as a
next step.

Thanks,
Bruce

Going back to 100LL probably contributed to the problem. Lead
deposits build up on the valve stems and inside guides causing them to
stick. Unleaded fuels are one reason cars last so long these days.

First check your engine for intake leaks (and for exhaust leaks while
you're at it) The rubber hose sections on the intake runners are
notorious for developing leaks. One way is to pressurize the intake
system with the blow side of a very clean shop vacuum (new is good)
and spray the outside of the engine with a soap solution looking for
bubbles.

If the intake system is tight, your suspicions on sticking valves may
be correct. You'll need to read up on the infamous "Lycoming rope
trick" which reads in part: "Insert about 8 feet of 3/8 inch nylon
rope through the spark plug hole; then turn the crankshaft until the
piston moves the rope snugly against the exhaust valve."

See: http://www.lycoming.com/support/publ...fs/SI1425a.pdf

This procedure allows the valves to be removed from their guides and
cleaned without removing the cylinders from the engine. You should
also clean the valve guides while you're at it. Be prepared to find
bad things that lead to cylinder replacement.

In the event this doesn't solve the problem, I'd look at the
possibility a magneto has gone intermittent. Mags have a service life
of about 500 hours but almost nobody replaces them until they fail -
and they can fail in some weird ways. In my airplane days, I replaced
mags every 500 hours with a new ones - it was my butt in the left seat.

Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. We are having intermittent problems with power surges that
can be significant and are sometimes even noticeable from the ground. The
problem will occur one day on one tow, and then sometimes disappear for
several weekends. One tow pilot seems to think that the problem seems to
appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for obstructions,
and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might have
started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses occur.
I have never experienced one of the major losses myself, but I have had a
few of the 100 rpm drops which cleared up after about 15 seconds or so.
When it happened to me I did notice a very slight engine roughness which
does sound like it could be a stuck valve. It also makes sense that if it
is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need our
Pawnee back on the flight line. We are close to doing the top overhaul as a
next step.

Thanks,
Bruce

On Apr 29, 1:07*pm, "bds" wrote:
"Tony" wrote

wing tanks or header tank? *has there been any consistency in fuel
level during these events? ive had engine problems in airplanes with
header tanks at extreme nose up attitudes and low fuel levels. *your
"negative G" towpilot could be on to something. is there an engine
driven fuel pump on the pawnee, even with a header tank? *have you
checked to make sure it is working?

ive had a valve stick (on a cessna 150) and it was VERY noticeable,
not "very slight" roughness.

*********************

It has a header tank but the problem occurs regardless of fuel level. *If
anything, it is almost always full or at least half full when the problem
occurs.

I think part of the reason that the roughness isn't as bad on the 0-540 is
because you're losing 1 out of 6 instead of 1 out of 4 when a valve sticks.

Bruce

fair enough. does it happen only on tow or sometimes off tow too?
one towpilot mentioned negative G's as a possible trigger. I wouldn't
rule out fuel delivery issues. with a header tank, possibly coupled
with a weak or not working engine driven fuel pump (assumption), you
could have issues when a glider pilot starts to yank the tail up or if
your towpilots like to dive away after tow release.

we had a similar problem years ago with a Pawnee 150.....would usually run
up fine on the ground but every couple of flights just after take-off when
you were expecting it to get up to full RPM it would only get about 80% of
the normal power ....unfortunately this was about the time you'd be lifting
off with a glider on behind.....the mechanic replaced carburetor float,
changed plugs, adjusted magneto's .all resulted in the same behavior...then
finally we checked the exhaust and sure enough a baffle had broken off
inside and sometimes would lay harmlessly away from the muffler inlet or
exit and other times would just simply end up plugging the exhaust end of
the muffler.....what goes in has to come out or less can come in and
consequently when less comes in and out the motor doesn't work as
intended...you might also notice a higher than normal engine temperature as
an indication that this is the problem ...anyway, this is my bet!
tim

Please visit the Wings & Wheels website at www.wingsandwheels.com



"bds" wrote in message
...
Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. We are having intermittent problems with power surges
that can be significant and are sometimes even noticeable from the ground.
The problem will occur one day on one tow, and then sometimes disappear
for several weekends. One tow pilot seems to think that the problem seems
to appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for
obstructions, and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might
have started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses
occur. I have never experienced one of the major losses myself, but I have
had a few of the 100 rpm drops which cleared up after about 15 seconds or
so. When it happened to me I did notice a very slight engine roughness
which does sound like it could be a stuck valve. It also makes sense that
if it is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need
our Pawnee back on the flight line. We are close to doing the top
overhaul as a next step.

Thanks,
Bruce



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The message was checked by ESET NOD32 Antivirus.

http://www.eset.com

What do you mean by a power surge? To me a surge is a momentary
increase in something. Are you experiencing a momentary power
increase, or a momentary power reduction?

The PA28-180 I used to be a partner in experienced a few 200 rpm drops
at the most inconvenient times. It scared off 3 of the partners and
it took several years for me to find the problem. I became more
motivated after it happened to me during a high density altitude
takeoff.

It was caused by a P lead contact being shorter than spec. The P lead
contact pushes against the contact spring mounted on the mag
capacitor. The short contact didn't push the spring contact far
enough away from the rear mag cover and allowed it to sometimes arc to
the case. Dismantled the P lead contact and replaced the spacer tube
with a longer one from a spare P lead (same length as the other mag)
and the power loss problem has not been seen in the last 3 or more
years.

New mags, or even a complete engine rebuild, would not have fixed the
problem unless the P leads were also replaced. This wouldn't normally
be done since they are part of the aircraft wiring harness.

Really bad mag design since if the P lead comes unscrewed the mag is
shorted internally and behaves just as if the P lead is grounded.

Andy (GY)

"Tony" wrote

wing tanks or header tank? has there been any consistency in fuel
level during these events? ive had engine problems in airplanes with
header tanks at extreme nose up attitudes and low fuel levels. your
"negative G" towpilot could be on to something. is there an engine
driven fuel pump on the pawnee, even with a header tank? have you
checked to make sure it is working?

ive had a valve stick (on a cessna 150) and it was VERY noticeable,
not "very slight" roughness.

*********************

It has a header tank but the problem occurs regardless of fuel level. If
anything, it is almost always full or at least half full when the problem
occurs.

I think part of the reason that the roughness isn't as bad on the 0-540 is
because you're losing 1 out of 6 instead of 1 out of 4 when a valve sticks.

Bruce

"bildan" wrote

Going back to 100LL probably contributed to the problem. Lead
deposits build up on the valve stems and inside guides causing them to
stick. Unleaded fuels are one reason cars last so long these days.

First check your engine for intake leaks (and for exhaust leaks while
you're at it) The rubber hose sections on the intake runners are
notorious for developing leaks. One way is to pressurize the intake
system with the blow side of a very clean shop vacuum (new is good)
and spray the outside of the engine with a soap solution looking for
bubbles.

If the intake system is tight, your suspicions on sticking valves may
be correct. You'll need to read up on the infamous "Lycoming rope
trick" which reads in part: "Insert about 8 feet of 3/8 inch nylon
rope through the spark plug hole; then turn the crankshaft until the
piston moves the rope snugly against the exhaust valve."

See:
http://www.lycoming.com/support/publ...fs/SI1425a.pdf

This procedure allows the valves to be removed from their guides and
cleaned without removing the cylinders from the engine. You should
also clean the valve guides while you're at it. Be prepared to find
bad things that lead to cylinder replacement.

In the event this doesn't solve the problem, I'd look at the
possibility a magneto has gone intermittent. Mags have a service life
of about 500 hours but almost nobody replaces them until they fail -
and they can fail in some weird ways. In my airplane days, I replaced
mags every 500 hours with a new ones - it was my butt in the left seat.

******************

We did check the intake tubes and couplers and tighten up the clamps but we
haven't done anything like the test with the vacuum cleaner - that sounds
like a good idea.

I recall reading something about the Lycoming rope trick last year when we
had the stuck valve on the one cylinder. I am thinking that if we had one
stick, it is reasonable to assume that we likely have problems with others.

Bruce

"Tim Mara" wrote in message
...
we had a similar problem years ago with a Pawnee 150.....would usually run
up fine on the ground but every couple of flights just after take-off when
you were expecting it to get up to full RPM it would only get about 80% of
the normal power ....unfortunately this was about the time you'd be
lifting off with a glider on behind.....the mechanic replaced carburetor
float, changed plugs, adjusted magneto's .all resulted in the same
behavior...then finally we checked the exhaust and sure enough a baffle
had broken off inside and sometimes would lay harmlessly away from the
muffler inlet or exit and other times would just simply end up plugging
the exhaust end of the muffler.....what goes in has to come out or less
can come in and consequently when less comes in and out the motor doesn't
work as intended...you might also notice a higher than normal engine
temperature as an indication that this is the problem ...anyway, this is
my bet!
tim

Please visit the Wings & Wheels website at www.wingsandwheels.com

We did find problems with the baffling inside the muffler over the winter
and thought it was our "aha" moment. The muffler was rebuilt but the
problem remains.

Bruce

"Andy" wrote in message
...
What do you mean by a power surge? To me a surge is a momentary
increase in something. Are you experiencing a momentary power
increase, or a momentary power reduction?

The PA28-180 I used to be a partner in experienced a few 200 rpm drops
at the most inconvenient times. It scared off 3 of the partners and
it took several years for me to find the problem. I became more
motivated after it happened to me during a high density altitude
takeoff.

It was caused by a P lead contact being shorter than spec. The P lead
contact pushes against the contact spring mounted on the mag
capacitor. The short contact didn't push the spring contact far
enough away from the rear mag cover and allowed it to sometimes arc to
the case. Dismantled the P lead contact and replaced the spacer tube
with a longer one from a spare P lead (same length as the other mag)
and the power loss problem has not been seen in the last 3 or more
years.

New mags, or even a complete engine rebuild, would not have fixed the
problem unless the P leads were also replaced. This wouldn't normally
be done since they are part of the aircraft wiring harness.

Really bad mag design since if the P lead comes unscrewed the mag is
shorted internally and behaves just as if the P lead is grounded.

Andy (GY)

Sorry - it is a sudden loss, not a sudden increase. Poor wording.

Thanks for the tip - we will take a look at this this weekend.

Bruce