Excessive valve clearance cause low power?

Hey all. Been meaning to post this for awhile (almost done with the annual now), but campus news
server has been down. Anyway...

I've got a 180hp O-360-A3A Lycoming in my Cherokee. The engine has always been just a bit weak
since we've owned it. We've now got about 500 hours on it since we bought it with what I now know was a
"service limits overhaul." It's always been at the very bottom end of the static RPM allowance on the
TCDS (2250 RPM is the bottom of the range). Very occasionally (approx 1 out of 25 takeoffs), the power
output is even more decreased for the first 60 seconds after takeoff. Not enough to notice under normal
conditions ("Geez... only getting 500 fpm instead of 650"), but definately enough to get your attention
at high density altitudes and/or heavily loaded. While it's not always obvious it's happening, when I
suspect it is I've done mag checks, looked at EGT, looked at manifold pressure.... nothing unusual and
everything is running smoothly.

Things we've checked over the years:
- Compression is always good (77-80/80 on all four)
- Propeller is correct and brand new when we bought it... pitched the standard 60"
- Tach has been checked with calibrated strobe-tach.
- Muffler baffles checked and fine... nothing loose inside
- Mags have had fresh points/condensers, internally and externally timed.
- Plugs have been replaced, cleaned, gapped, rotated multiple times, etc.
- Done "wobble test" on valve guides. Worn, but within the limits.

So, while we were doing the annual this year, we figured we'd check to make sure the hydraulic
lifters were OK. They checked out fine, but when we put it back together noticed that a number of the
valves had excessive dry-tappet clearance. Book calls for 0.028-0.080. We had only two barely within
that spec, and 6 with clearances ranging from 0.090-0.130. We've got some longer pushrods on the way to
get them all within spec.

The question is, will the hydraulic lifters take up the slack, or could the longer extension
cause increased bleed-down that would reduce valve travel? If the former, then it would just make the
valves noisy for a bit after startup, but takeoff/climb/cruise would be unaffected. If the latter, then
maybe it would affect the power at all RPMs?

Another thing we checked while we had the lifters out was the cam lift. Putting a dial indicator
on the tappets directly, we measured anywhere from 0.355-0.360" on all lobes. After calling a half-dozen
shops to find out what the book specification is for a Lycoming cam (apparently not published anywhere),
I measured a friend's new ECI AEL-18840 cam at 0.353. Since all of my lobes are the same, and apparently
the same as a new ECI cam, I figure the cam is fine.

Thoughts? Comments? If this fixes the chronic low power, I'll be ecstatic, but I'm skeptical that it
will affect anything.

-Cory



--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

On Sep 25, 6:53 am, wrote:
Hey all. Been meaning to post this for awhile (almost done with the annual now), but campus news
server has been down. Anyway...


The dry tappet clearance might be part of the problem.
The clearances can increase as the cylinder lengthens with heat, and a
few valves might not be opening all the way. But it should be a
consistent lack, not 1 out of 25 takeoffs.
I would suspect that carb icing might be your problem.
It's way more common than many folks think. What are the atmospheric
conditions when this happens? Look for small temperature/dewpoint
spreads. It doesn't have to be cold outside; ice can appear at temps
up to 100°F. Lycomings are ice-resistant because their carbs are
mounted on the oil sump, but if the oil isn't really hot yet, or if
the temps are cooler and the dewpoint isn't far away, ice will form.
Sometimes even if the oil's hot, and even in the full-throttle climb.
Does the engine stumble once in a while in the climb? Probably ice. We
see ice all the time here, even in a dry climate and on Lycomings and
on warm days. Got to know what to watch for.
Check the carb heat immediately before takeoff, at around
12 or 1400 RPM. Should get a drop that stays dropped; any rise after
that with carb heat on, or excessive roughness before it rises,
indicates ice. There have been accidents where engines didn't generate
enough power to sustain a climb, and the cause was pegged as ice that
formed during the taxi, after the runup.

Dan

: The dry tappet clearance might be part of the problem.
: The clearances can increase as the cylinder lengthens with heat, and a
: few valves might not be opening all the way. But it should be a
: consistent lack, not 1 out of 25 takeoffs.

I may not have been completely clear in my zeal to describe my frustration... There are two
power issues we're trying to resolve. There's a chronic loss of power (never made more than the very
bottom of the static RPM). The occasional loss is so subtle, infrequent, and irreproduceable, that's
it's very difficult to track down. We thought maybe gunk in one of the lifters that occasionally
prevented pumping up which is why we decided to look. I'm now wondering if the 1-in-25 thing is the
lifter spring snapping out of its little shoulder due to the excessive clearance. Then it might
occasionally snap back in.

: I would suspect that carb icing might be your problem.
: It's way more common than many folks think. What are the atmospheric
: conditions when this happens? Look for small temperature/dewpoint
: spreads. It doesn't have to be cold outside; ice can appear at temps
: up to 100?F. Lycomings are ice-resistant because their carbs are
: mounted on the oil sump, but if the oil isn't really hot yet, or if
: the temps are cooler and the dewpoint isn't far away, ice will form.
: Sometimes even if the oil's hot, and even in the full-throttle climb.
: Does the engine stumble once in a while in the climb? Probably ice. We
: see ice all the time here, even in a dry climate and on Lycomings and
: on warm days. Got to know what to watch for.
: Check the carb heat immediately before takeoff, at around
: 12 or 1400 RPM. Should get a drop that stays dropped; any rise after
: that with carb heat on, or excessive roughness before it rises,
: indicates ice. There have been accidents where engines didn't generate
: enough power to sustain a climb, and the cause was pegged as ice that
: formed during the taxi, after the runup.

I've gotten carb ice a few times... even in fairly warm temps. I'm skeptical that this is the
cause because it doesn't seem to be temperature related. It's done it on nice cold (35) dry, winter
days, as well as hazy hot (95) summer days. Also, I've got a manifold pressure gauge on this plane even
though it's fixed pitch. Manifold pressure indicates correctly when it's acting up... if it were carb
ice it'd indicate less MP.

I'm glad to hear someone agreeing that chronic loss might be due to valve clearance.

-Cory


--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:

I would suspect that carb icing might be your problem.
It's way more common than many folks think. What are the atmospheric
conditions when this happens? Look for small temperature/dewpoint
spreads. It doesn't have to be cold outside; ice can appear at temps
up to 100°F. Lycomings are ice-resistant because their carbs are
mounted on the oil sump, but if the oil isn't really hot yet, or if
the temps are cooler and the dewpoint isn't far away, ice will form.
Sometimes even if the oil's hot, and even in the full-throttle climb.
Does the engine stumble once in a while in the climb? Probably ice. We
see ice all the time here, even in a dry climate and on Lycomings and
on warm days. Got to know what to watch for.
Check the carb heat immediately before takeoff, at around
12 or 1400 RPM. Should get a drop that stays dropped; any rise after
that with carb heat on, or excessive roughness before it rises,
indicates ice. There have been accidents where engines didn't generate
enough power to sustain a climb, and the cause was pegged as ice that
formed during the taxi, after the runup.

http://www.ez.org/carb_ice.htm

I don't know whether carb ice is Cory's problem, but I agree many
Cherokee pilots don't appreciate the probability of carb ice, probably
due to Piper's checklist not specifying carb heat for all landings /
descents as Cessna's checklist does. You *can* get carb ice in a
cherokee. Watch out during those long, slow instrument approaches
through visible moisture. Sorry for the slight thread misdirection.

Dave

How old are your sparkplugs? We've had a lot of trouble over the
years with Champions, and used to buy the Auburns before Champion
bought them up and shut them down. They were an excellent plug and
almost always eroded to limits before the internal resistor failed.
With Champs, we've had the internal resistor misfiring at 50 hours.
One didn't work right out of the box. The Unison plugs are no better.
A poorly-firing plug will cause a slight RPM drop. They need
to be tested under pressure to see that they continue to fire up to at
least 120 psi.

Dan

wrote:

: How old are your sparkplugs? We've had a lot of trouble over the
: years with Champions, and used to buy the Auburns before Champion
: bought them up and shut them down. They were an excellent plug and
: almost always eroded to limits before the internal resistor failed.
: With Champs, we've had the internal resistor misfiring at 50 hours.
: One didn't work right out of the box. The Unison plugs are no better.
: A poorly-firing plug will cause a slight RPM drop. They need
: to be tested under pressure to see that they continue to fire up to at
: least 120 psi.

It's done the intermittent thing with new plugs, as well as freshly gapped and cleaned. Besides, if a plug wasn't
firing, the mag drop would show it (it doesn't... runs smoothly on each mag by itself).

Nice thought though. We've already done all the easy and normal stuff....

-Cory


--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
wrote:

: How old are your sparkplugs? We've had a lot of trouble over the
: years with Champions, and used to buy the Auburns before Champion
: bought them up and shut them down. They were an excellent plug and
: almost always eroded to limits before the internal resistor failed.
: With Champs, we've had the internal resistor misfiring at 50 hours.
: One didn't work right out of the box. The Unison plugs are no better.
: A poorly-firing plug will cause a slight RPM drop. They need
: to be tested under pressure to see that they continue to fire up to at
: least 120 psi.

It's done the intermittent thing with new plugs, as well as freshly gapped and cleaned. Besides, if a plug wasn't
firing, the mag drop would show it (it doesn't... runs smoothly on each mag by itself).

Nice thought though. We've already done all the easy and normal stuff....

-Cory


Exhaust restriction? Bad mufflers can rob lots of power and be
intermittent.
I'm not saying valve clearance isn't the problem, but your clearances
don't seem extreme.
Also, plugs can test well but the internal resistance can bugger things
up. I've read that 5k ohms or over may be excessive. The spark will find
easier places to go than across the electrodes.
good luck

Dave

Dave wrote:
: Exhaust restriction? Bad mufflers can rob lots of power and be
: intermittent.
: I'm not saying valve clearance isn't the problem, but your clearances
: don't seem extreme.
: Also, plugs can test well but the internal resistance can bugger things
: up. I've read that 5k ohms or over may be excessive. The spark will find
: easier places to go than across the electrodes.
: good luck

As I said in my initial message, muffler baffles checked and fine. I have overhauled the mufflers this year due to
cracks in the end plates, but the baffles were fine when they were removed.

As far as the plugs, we replaced them about a year ago. Had the intermittent failures with the old plugs, new plugs,
and mag drops were fine in all cases. Doesn't seem to be ignition-related.

-Cory



************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

On Sep 25, 8:46 pm, wrote:
Dave wrote:

: Exhaust restriction? Bad mufflers can rob lots of power and be
: intermittent.
: I'm not saying valve clearance isn't the problem, but your clearances
: don't seem extreme.
: Also, plugs can test well but the internal resistance can bugger things
: up. I've read that 5k ohms or over may be excessive. The spark will find
: easier places to go than across the electrodes.
: good luck

As I said in my initial message, muffler baffles checked and fine. I have overhauled the mufflers this year due to
cracks in the end plates, but the baffles were fine when they were removed.

As far as the plugs, we replaced them about a year ago. Had the intermittent failures with the old plugs, new plugs,
and mag drops were fine in all cases. Doesn't seem to be ignition-related.


One more thing: Lycomings, especially those built or
rebuilt before 1998, have a habit of wearing their exhaust valve
guides enough that the valve can start seating off center and leaking.
Normally this will show up as an exhaust leak on a differential
compression test, but sometimes not until the wear is pretty bad. As
the valve strikes the seat off-center, it hangs there briefly before
closing and some of the compression is lost.
See http://www.lycoming.com/support/publ...dfs/SB388C.pdf

Dan

I've have discovered just-barely-out-of-limits pushrod lengths on one
cylinder can cause a loss of 2-3 knots cruise speed.

Excessive clearance changes the effective valve timing, retarding the
opening and thus decreasing power.
This is one area that blueprint engine builders pay much attention to.

I have a suspicion that since the field overhaul ignored the pushrod
lengths, they probably ignored the testing of the lifter hydraulic units.
There is a spec for the bleed-down rate.

You need to fix this soon. Excessive clearance is hard on the camshaft.

I know where find the camshaft specs. I'll post them here.

Kent Felkins
Tulsa



wrote in message
...
Hey all. Been meaning to post this for awhile (almost done with the
annual now), but campus news
server has been down. Anyway...

I've got a 180hp O-360-A3A Lycoming in my Cherokee. The engine has always
been just a bit weak
since we've owned it. We've now got about 500 hours on it since we bought
it with what I now know was a
"service limits overhaul." It's always been at the very bottom end of the
static RPM allowance on the
TCDS (2250 RPM is the bottom of the range). Very occasionally (approx 1
out of 25 takeoffs), the power
output is even more decreased for the first 60 seconds after takeoff. Not
enough to notice under normal
conditions ("Geez... only getting 500 fpm instead of 650"), but definately
enough to get your attention
at high density altitudes and/or heavily loaded. While it's not always
obvious it's happening, when I
suspect it is I've done mag checks, looked at EGT, looked at manifold
pressure.... nothing unusual and
everything is running smoothly.

Things we've checked over the years:
- Compression is always good (77-80/80 on all four)
- Propeller is correct and brand new when we bought it... pitched the
standard 60"
- Tach has been checked with calibrated strobe-tach.
- Muffler baffles checked and fine... nothing loose inside
- Mags have had fresh points/condensers, internally and externally timed.
- Plugs have been replaced, cleaned, gapped, rotated multiple times, etc.
- Done "wobble test" on valve guides. Worn, but within the limits.

So, while we were doing the annual this year, we figured we'd check to
make sure the hydraulic
lifters were OK. They checked out fine, but when we put it back together
noticed that a number of the
valves had excessive dry-tappet clearance. Book calls for 0.028-0.080.
We had only two barely within
that spec, and 6 with clearances ranging from 0.090-0.130. We've got some
longer pushrods on the way to
get them all within spec.

The question is, will the hydraulic lifters take up the slack, or could
the longer extension
cause increased bleed-down that would reduce valve travel? If the former,
then it would just make the
valves noisy for a bit after startup, but takeoff/climb/cruise would be
unaffected. If the latter, then
maybe it would affect the power at all RPMs?

Another thing we checked while we had the lifters out was the cam lift.
Putting a dial indicator
on the tappets directly, we measured anywhere from 0.355-0.360" on all
lobes. After calling a half-dozen
shops to find out what the book specification is for a Lycoming cam
(apparently not published anywhere),
I measured a friend's new ECI AEL-18840 cam at 0.353. Since all of my
lobes are the same, and apparently
the same as a new ECI cam, I figure the cam is fine.

Thoughts? Comments? If this fixes the chronic low power, I'll be
ecstatic, but I'm skeptical that it
will affect anything.

-Cory



--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************