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

BTW -

Check that your cylinder baffling system is complete, tight to the
cylinders and properly secured to the engine, and that air loads don't
cause it to deflect away from the fins. You seem to have had a lot of
cylinder agony.

Do you have a balance problem? Above 1700 RPM the engine mounts may be
able to isolate a still-running-rough engine from the cockpit.

Be sure the plug gaps are small enough that you don't cause a marginal
ignition harness to flashover at higher power.

Does carb heat smooth things out? If so the engine is running lean.

It could also be an intake manifold gasket that has sucked in, except
that I wouldn't expect the idle to be smooth.

NRPetersen