Looking at a 1991 boat with a 1991 Mercury.
Anybody have any experience with those motors?

Is it a known "good" motor or a known "dog"

It had the gear box (or bottom part of the lower unit) replaced two years ago.

The guy said something about that it is supposed to run on different number of cylinders depending on if you are idling or going fast or something. Sounded too complicated for me.