Mystery Virbration - Part 2

The "Mystery Vibration" saga in the 74 Cherokee 140 (O-320 -
E3D)continues. I began the quest by addressing those items that were due
anyway. Swapped out the plugs with the old set and saw no difference.
Had the mags overhauled as they were 20 hours away from their 500 hour
mark where I usually take them in. Replaced the harnesses because they
were 6 years old and showing some signs of age. No difference. I DO get
a 150 rpm drop on the left mag with only 50 on the right. That appeared
to be the case before and after overhaul. It seems to run a bit rough on
the left, but that may just be my perception because the drop brings it
to a lower RPM. Just for grins, I swapped the plugs between the mags,
but saw no difference.

I did know the #1 cylinder was going soft. The last 2 compression
readings were not stellar. Test reading last week showed it at 57/80
hissing out the exhaust. So, I hauled the thing down to the wrench
yesterday with yet another (third one in 900 hours) P10 class 4 straight
bore plain steel overhaul in tow (complete with valves). Ironically, on
the trip over, it ran almost perfectly. I felt only 1 small "burble" in
30 flying minutes. Swapped out the cylinder in just under 5 hours
including an "insurance" compression check, air filter change,
alternator belt snug up, and tail cone clean out (damn birds). On the
trip home, the vibration returned with a vengeance. The difference
between the two trips was that the trip out was in air like glass with
the return in moderate bumps. I call turbulence "moderate" when I feel
the pressing need to land because flying is no longer any fun.

Since this may be airframe or control surface related, I opened up the
inspection covers and checked all the control cables and linkages.
Nothing unusual anywhere (I HATE going back into that tail).

I did notice the carb throttle shaft seemed to have a bit of wiggle. Not
sure if that may be a factor. The wrench (52 years of engine rebuilding)
said it was not a factor.

I guess I'll fly it for a few hours until #1 breaks in and then have the
prop balanced. I wanted to get that done as it has been about 1300 hours
or so since it was last dynamically balanced.

On another note, with the #1 cylinder removed, I got a great look at all
the cam lobes. As a possible vibration source, I can eliminate a bad cam
lobe.

Other than that, I am out of ideas. I may have to fly it and wait for a
more remarkable sign. Hopefully that won't be when something significant
happens at an inconvenient time.

Mike

You didn't mention if the vibration is occurring during cruise at
constant RMP.

john smith wrote:
You didn't mention if the vibration is occurring during cruise at
constant RMP.

Yes. It vibrates during cruise at constant RPM. If I climb up to around
4500 feet or so, it diminishes or disappears. Makes me think it may be
an intake leak.
Mike