A spark plug that is broken internally will cause this at high power
settings. Find a spark plug tester that uses air pressure and fires
the plugs under pressure to test the plugs.

A bad plug wire or a mag that is arcing internally will also cause a
problem.

A quick mag bearing check is to do a mag check at idle rpm. If the
RPM drops are not the same and it is rough then the mag bearings are
loose and the rotor is flopping around changing the timing.

John

On Thu, 11 Nov 2004 05:23:09 GMT, "R&A Kyle"
wrote:

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