Stuck valve.. Check that happens a lot..

Troy
"Fly" wrote in message ...
I've balanced many props, and also owned a '75 8kcab for about 500 hours.

Key point of yours is the fact that the vibration isn't there all the
time.
My gut feeling and psuedo-internet expert guess?
Airframe....but then maybe not.

If it is engine related, it'll be ignition or valve-train. But verify the
usual suspects also, compressions mag timing and so.
The hydralic units in the lifter bodies can go soft, bypassing more oil
pressure.
This changes the effective valve timing.

Do a mag check during cruise, this is where bad plugs or faults will show
up.
A mag check at 1700 basically just shows if the P-leads are grounded.
Also, make sure there is not interference with parts rubbing the airframe.
Exhaust pipes, hose clamps, baffling.....

Airframe, check for slop in the trim tab and other controls. Check the
bushings etc. Check the flying wire tension. Cable tensions etc.

Frankly, many aircraft can pass annual but could still benefit if time and
labor was spent going thru the tail section and getting everything up to
spec. Small annoying cockpit vibrations can come from many sources.

A Bonaza owner noticed his MP guage smoothed after removing an air skeg on
his tail.

I diagnosed trim-tab flutter in a Belllanca Viking that caused a cyclic
buzz
to travel thru the airframe and up into the seats. Trim-tab had a
half-inch
of slop.
A commanche had a shark fin antenna on top just aft of the baggage
compartment. It had been installed without a doubler. It fluttered 2-3
inches. It was cracking the skin.

Once, the aluminum strip above my 8KCAB windshield come loose, the rubber
nuts pulled out, and the vibration was scary!

How many pilots have really tried to look at the tail and other parts in
flight?
You gotta bend around and press your face up against the glass.
Get a buddy to fly formation for a looksee.

And if it hasn't been done....anyway definitely get a dynamic prop balance
check!!!
88% of prop aircraft can be improved.

Try to find a technician that has experience. Put two sensors on the
engine, fore and aft.

Adding a correction weight to a Lycoming ring-gear is an approved method,
but I have learned to shy away from that method unless it is the last
resort.
I just never been happy doing it that way.
It usually requires much more correction weight per unit of vibration and
it
usually not in the optimum correction plane.

I charge $175-200. I don't do it for less. My $$$ equipment,
experience
and service is what you get. If I can't help it, my fee is less.
And actually, the balance is is generally good for 600-1000 hours or
maybe
more, so how does that prorate out?

let us know what you find out.

regards,

Kent Felkins
Felkins Aircraft
Tulsa Oklahoma










"TIm Gilbert" wrote in message
...
We have a '67 KCAB that has a weird vibration from time to time. You
can
feel it in the stick and seat. It is the worst around 2200-2400 rpm but
only in the air. On the ground it isn't noticeable. Oil analysis is
fine
and the engine seems smooth. We've installed new motor mounts and
everything else on the airframe seems tight. One suggestion, from a
local
KCAB expert, is that it is from the crossover exhaust. We've considered
having the prop balanced but figure that if it was a prop problem it
would
occur all the time. Maybe a sticky valve but I would expect a sticky
valve
to be 'cold engine' problem.

Any ideas?

Tim
N2606G