Ignition timing is so fundamental to the proper operation of a piston
engine that it should never be taken for granted. Serious owners
should really learn how to check it themselves. Theoretically they are
not permitted by FARs to adjust it, but they are certainly able to
check it.

The Continental O-300D is about the smoothest running engine in
aviation. It has the torsional damper masses on the rear of the
crankshaft that the earlier O-300s don't have.