I have a 140 L that has the following symptoms when warmed up (cold it seems fine except for the initial full throttle run). In the mid range and idle (warm now) it surges and smokes a lot as though rich, and after going to full throttle (sometimes even the first time) there is a noticeable drop in RPM after a few (3-5) seconds. The engine also doesn't seem to have as much power as it used to. Am I looking at the "dreaded" piston rock? If it is that, how far do I go with replacing parts? Just Piston; Piston and Ring; Piston and Liner; Piston, Ring, and Liner?

Any thoughts on a path to take?

Thanks and keep up the good work!
Tom

Hard to tell without seeing it. At a minimum, the ring needs replacing, but it could need a ring, piston, cylinder and who knows what else.

If you can't tell yourself, I would send it to YS Service,as it may save you money just trying to chase it with parts.