Yes, I understand that part of it. The stbd engine doesn't have a thermostat in it at all and it smokes white for a few minutes until it warms up. I would imagine a thermostat stuck open would cause the same effects.
What I'm seeing is an engine that starts and runs just fine for 1-2 hours and then starts running rough and emitting puffs of whitish/grayish smoke. It's not blue, and it's not black, so I guess it's white.
I was curious as to how you can tell an engine is overheated by observing operation, without any gauges to read temp.
I checked impeller yesterday, it's good. Like new. I did find the water
pump belt was a bit on the loose side, and tightened that. I ran for amost four hours today at reduced RPM
and it didn't miss a beat.