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



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