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

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