Excessive valve clearance cause low power?

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