we had a similar problem years ago with a Pawnee 150.....would usually run
up fine on the ground but every couple of flights just after take-off when
you were expecting it to get up to full RPM it would only get about 80% of
the normal power ....unfortunately this was about the time you'd be lifting
off with a glider on behind.....the mechanic replaced carburetor float,
changed plugs, adjusted magneto's .all resulted in the same behavior...then
finally we checked the exhaust and sure enough a baffle had broken off
inside and sometimes would lay harmlessly away from the muffler inlet or
exit and other times would just simply end up plugging the exhaust end of
the muffler.....what goes in has to come out or less can come in and
consequently when less comes in and out the motor doesn't work as
intended...you might also notice a higher than normal engine temperature as
an indication that this is the problem ...anyway, this is my bet!
tim
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"bds" wrote in message
...
Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. We are having intermittent problems with power surges
that can be significant and are sometimes even noticeable from the ground.
The problem will occur one day on one tow, and then sometimes disappear
for several weekends. One tow pilot seems to think that the problem seems
to appear any time the airplane experiences noticeable negative Gs.
In the past year or so we have replaced or rebuilt the following:
Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs
We have also cleaned the gascolator, checked the fuel tank for
obstructions, and checked the vent system.
The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might
have started or gotten worse shortly afterwards.
I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses
occur. I have never experienced one of the major losses myself, but I have
had a few of the 100 rpm drops which cleared up after about 15 seconds or
so. When it happened to me I did notice a very slight engine roughness
which does sound like it could be a stuck valve. It also makes sense that
if it is sticky valves that the problem would get worse running on 100LL.
Any comments or suggestions on what to do next? Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need
our Pawnee back on the flight line. We are close to doing the top
overhaul as a next step.
Thanks,
Bruce
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