I'd be suspicious of airframe something. Like trim tab flutter.
Doublecheck the various tail bushings and such. Its an old airframe.

A sporadic engine related vibration is either ignition or valve train. Do
a mag check at cruise power. Check valve springs.

Kent Felkins



Engine vibration will usally be steady state
"R&A Kyle" wrote in message
news:GIsld.3601$d96.2476@trnddc01...
First, thanks for all these comments. The roughness is enough to cause a
low level of vibration in the airframe; not enough to terminate the
flight,
but the needles do dance. Another IA, not familiar with this plane, made
a
short flight and agreed. The roughness is not steady, but rather comes in
waves, 3 or 4 seconds of rough followed by a slightly shorter interval of
somewhat smoother operation. (I realize this is very subjective.)

We systematically changed mix, MP, RPM, carb heat, mags and primer pump
(lock/unlock). Roughness appears only above 20 in MP. No correlation
with
other inputs.
"Fly" wrote in message
...
How are you perceiving the roughness?

How you know the engine mounts are OK?

Kent Felkins
Tulsa



"R&A Kyle" wrote in message
news:1HCkd.12833$z_4.9667@trnddc07...
Can anyone suggest a cause for a slight roughness in a well maintained
O-470
(1958 C-182). The mechanic has checked both mags and harness wires
and
plugs, installed a rebuild carb and balanced the prop (which required
very
little correction). In flight checks of carb heat, mags and unlocking
the
primer pump all show no effect. Roughness appears above 20" MP at any
RPM.
Engine mount checks ok.

Any suggestions would be welcome.
R Kyle