Does the best ROC indicated airspeed change with the engine and prop
upgrade? If it did and you are flying the old numbers, perhaps that is
affecting your performance.

Regards,
Mike.

http://flickr.com/photos/mikenoel/

http://photoshow.comcast.net/mikenoel


wrote in message
...
Hey all. As some of you may recall, about a year ago I posted a question
regarding two different power loss issues I was having
with my PA-28-140 with an O-360-A4A 180hp upgraded engine. I'll summarize
what's transpired and figure to get some opinions on what I
might try next.

Basically, I had an intermittent slightly low power on about 1 in 25
takeoffs. Not enough to make you abort, but enough to realize
upon climbout that it wasn't quite doing what it usually did in terms fo
FPM. Never rough, did mag checks, fuel pressure good, EGT
correct, CHT correct, etc, etc... just a little low on power, and after a
minute or so it would quietly come back. After checking cam
lift and lifters, etc, I realized that the dry tappet clearance was too
much on about 5 of the 8 pushrods. I fixed that, and the problem
has not come back after a year.

The *other* problem is that this engine has never been very strong. It's
always been barely able to make the minimum static RPM
required by the TCDS, and climbs out at about 2400 RPM at Vy. This
problem persists.... it feels more like a -160 than a -180 and I've
checked all sorts of things that would be common:

- Compressions great... 78/80-80/80 on all four chrome cylinders. No
hissing out exhaust or crankcase.
- Mags internally timed when we replaced the points and condensers, and
externally set at 25 degrees as per spec
- Does it with new plugs.
- Manifold pressure (added as part of the engine upgrade and measured at
the intake port on #3) is good for full-power takeoff... about
1-1.5" or so less than the ambient pressure (e.g. 30.00 altimeter here at
2100' MSL airport gives me about 26.5-27" MP on takeoff)
- EGT reads 1350 on all cylinders at full rich, so it's not starved for
fuel.
- Fuel pressure stays at 3-4 psi.
- Does it on 100LL or 93 AKI cargas or any mixture of the two.
- Did the "wobble test" SB on the valves. Acceptable, although somewhat
worn.
- Brand new exhaust system... no change
- Measured cam lift last year... all lobes show 0.353" +- 0.002"... not a
worn cam as that's the same as a friend's brand new Superior
cam.
- Propeller is stock, 60" pitch and brand new when we got the plane. I
believe it's 60" as other than full-power, the POH is dead-balls on
as far as airspeed and fuel burn are concerned. Any other pitch prop and
those wouldn't be right.

SO.... my thinking is this. I was skeptical that the dry tappet clearance
could affect the intermittent power problem, since the
hydraulic lifters should be able to compensate for this while running. I
appeared to be in error as fixing the dry tappet clearance to
spec seems to have cleared this up (took me a year to really believe
that). The original overhaul on this engine happened before we bought
it was definately a field overhaul... probably more or less a "service
limits" overhaul. Any chance that if the lifters are mismatched
from a shoddy overhaul that they'd bleed down too quickly and cause a
chronic low power from insufficient valve lift? If I could buy a new
set of lifters to fix the problem, I'd be a happy guy!

Any other ideas to entertain? I'm open to suggestions... this has been
p*ssing me off for literally *YEARS*!

Thanks,
-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D. Electrical Engineering, PPSEL-IA *
* Research Associate, Vibrations and Acoustics Laboratory *
* Mechanical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************