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