Excessive valve clearance cause low power?

On Sep 25, 6:34 pm, 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

The plugs will fire during the runup, one mag at a time, but
during flight when both mags are firing, there may be one or two that
don't. Engines that have both mags timed at the same point BTDC need
to be set as accurately as possible so that both plugs fire at the
same time or very close. If one fires well before the other, the
cylinder pressures may rise to a point that the other plug, being a
little weak, can't fire and now we get a bit of roughness and loss of
power.
There are a few engines that specify a significant difference
in their mag timing. I don't know how they handle spark blowout with
that setup. Some of them were pretty old models that used resistorless
plugs; perhaps they didn't suffer the same way that plugs with
resistors do.

Dan

I changed out one cylinder where the dry lash was out of spec even
with oversized pushrods. I had to find a newer rocker arm that had
more metal on the valve end.

: Nice thought though. We've already done all the easy and normal stuff....
:
: -Cory
:
: The plugs will fire during the runup, one mag at a time, but
: during flight when both mags are firing, there may be one or two that
: don't. Engines that have both mags timed at the same point BTDC need
: to be set as accurately as possible so that both plugs fire at the
: same time or very close. If one fires well before the other, the
: cylinder pressures may rise to a point that the other plug, being a
: little weak, can't fire and now we get a bit of roughness and loss of
: power.
: There are a few engines that specify a significant difference
: in their mag timing. I don't know how they handle spark blowout with
: that setup. Some of them were pretty old models that used resistorless
: plugs; perhaps they didn't suffer the same way that plugs with
: resistors do.

Interesting thought. I'm still skeptical that could be it, though. I've timed the mags a few
different times over the course of the year or so it's occasionally happened. I'm pretty anal about
getting them dead-nuts on each other and exactly at 25 degrees. I don't want them advanced at all since
I run autogas, but don't want to detune them either.

Also, I've done the mag drop in flight when I suspected that it was happening. Everything was
fine... power smoothly decreased as expected. The "blow-out" thing could still be doing it, but I
wouldn't think it would only do it for the first 60 seconds of flight occasionally. It should be more
consistent or reproducable with higher MP (lower altitude takeoffs), etc.

-Cory


--

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

Fly fly at Tulsaconnect.com wrote:
: 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

It sure seems like the lifters would have plenty of adjustment to take up the slack. The
bleed-down rate would have to be affected pretty significantly to affect the effective valve lift at 2500
RPM though, I'd think. From what I've been able to google up on the subject, bleed-down shouldn't affect
high-RPM valve operation much. It's pretty much a low-RPM thing. Then again, you can't believe
everything you read online... much better to appeal to random folks on USENET.

As far as the lifters go, we did the "squeeze" test as mentioned in the overhaul engine manual.
Nothing out of spec as far as that was concerned, but it's probably not too accurate.

If you could find the camshaft specs, it would be absolutely the only place they're posted on the
'net as far as I've been able to find. That'd be great...

-Cory

--

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

On Oct 9, 9:50 am, wrote:

Interesting thought. I'm still skeptical that could be it, though.
I've timed the mags a few
different times over the course of the year or so it's occasionally happened. I'm pretty anal about
getting them dead-nuts on each other and exactly at 25 degrees. I don't want them advanced at all since
I run autogas, but don't want to detune them either.

Also, I've done the mag drop in flight when I suspected that it was happening. Everything was
fine... power smoothly decreased as expected. The "blow-out" thing could still be doing it, but I
wouldn't think it would only do it for the first 60 seconds of flight occasionally. It should be more
consistent or reproducable with higher MP (lower altitude takeoffs), etc.

Another idea. We had a Cessna 150 that used to lose some
power on takeoff, sometimes shuddered while doing it, and after a lot
of messing around we found that the valve stem/guide clearance was too
small. This was a problem is the smaller Continentals and when the
rebuilders did those engines they had to be careful not to get the
guides too tight. Lycomings tend to wear their guides bigger, so the
clearances get worse, but a few have had sticking valves caused by
carbon buildup on the exhaust valve stems. The sticking would usually
cause a power loss with vibration, and if they stuck bad enough the
pushrod would bend, and bend the rod tube like so:
http://www.prime-mover.org/Engines/L...r/bentpush.jpg
You said you'd done the wobble test and the guides were
worn but within limits. None were tight, huh? The 150's sticking
valves were the intakes, not the exhausts, and the Lycoming wobble
check applies only to the exhaust valves.

Dan

New camhafts been measured at .357 intake and .358 exhaust lift. At .050
valve opening.


wrote in message
...
Fly fly at Tulsaconnect.com wrote:
: 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

It sure seems like the lifters would have plenty of adjustment to take up
the slack. The
bleed-down rate would have to be affected pretty significantly to affect
the effective valve lift at 2500
RPM though, I'd think. From what I've been able to google up on the
subject, bleed-down shouldn't affect
high-RPM valve operation much. It's pretty much a low-RPM thing. Then
again, you can't believe
everything you read online... much better to appeal to random folks on
USENET.

As far as the lifters go, we did the "squeeze" test as mentioned in the
overhaul engine manual.
Nothing out of spec as far as that was concerned, but it's probably not
too accurate.

If you could find the camshaft specs, it would be absolutely the only
place they're posted on the
'net as far as I've been able to find. That'd be great...

-Cory

--

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

Fly fly at Tulsaconnect.com wrote:
: New camhafts been measured at .357 intake and .358 exhaust lift. At .050
: valve opening.

Thank you. That's what I measured on all of the lobes of mine (at least to within the
0.001-0.003 accuracy I could measure with my dial indicator).

-Cory


--

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