Help; Rough running Continental O-300D

Hmmm, do you have a carb with a 1-piece venturi? On some applications these
made the engines run so poorly that the 2-pc was allowed to be reinstalled.

Does leaning, even leaning to the point of the engine dying, change the
roughness? Perhaps the carburettor is incorrectly jetted.

Did you look at the distributor blocks in the mags? Or the coils? Perhaps
the increased temperature of running at 1000-1700 with less cooling airflow
is making something conduct that shouldn't?
--
Aaron Coolidge

There in lies the mystery. It ran fine before the timing was set correctly.
Leaning has no effect. It is also only this RPM range. Outside of it, the
engine runs like a champ. The side effect of fixing the timing is that the
engine consistently runs 100 deg cooler on the head temp. I am wondering if the
plugs are reacting funny. If I somehow "partially" contaminated them when I had
them out for the cylinder repair. It doesn't, however, feel like they are
fouling since leaving the effected RPM range requires no "clearing".

Jim