I just had my 20 hour service on my new motor. I'm no mechanic, so I had it
done by the Sea Ray dealer. I mentioned to them that it had a difficult
first start once. Also it has been continuing to run ("dieseling") after
turning off the ignition. They said everything checked out, but they went
ahead and "advanced the timing a little." I should have kept my mouth shut,
but ...

Could be my imagination, the boat seems to run well, but it sure doesn't
seem to sound like it did before the 20 hour service. Instead of a nice
tight sounding motor it sounds like it's full of marbles. And I still had a
couple of episodes of "dieseling."

Why the "dieseling" before and after the service?

Does advancing the timing "a little" alter the sound of a motor?

Sea Ray 185
6 Cylinder carbureted 4.3L Alpha1 Mercruiser