Smoke in the cabin and my theory
This weekend, my 11 year old son and I went sailing. No wind in the morning so we motored (Farymann) for about 90 minutes. This is a new boat to me and while we had run the motor several times before without problem, this was the longest we'd run it. We also ran it pretty hard, keeping the speed up. I wanted to see how it did under load for a bit.
It ran great but after 90 minutes or so, my son noticed smoke/exhaust, something in the cabin. Not enough that you couldn't see through the cabin but enough so it'd be uncomfortable to breath.
I shut the engine off, he took the helm, and I went below to look for a fire by the engine...none was present. I opened some windows/hatches to clear the smoke out, then looked in the engine compartment again to see if I could see where it came from. Of course, with the engine off, I couldn't tell but got the general impression it was coming off a wide area of the front of the engine.
I was about to start the engine again to see if that would reveal where it came from but my son suggested lunch first. That way if the boat blew up we could tread water on a full stomach; an excellent idea.
After lunch, I fired up the engine, and it ran fine for a few minutes, with no evidence of any smoke anywhere. I shut it off again, and we sailed back toward home. When we got close, I fired it up again to dock and it ran for about 15 more minutes without any signs of smoke or trouble.
My theory: There's some grease/grime/oil on the surface of the engine. It was on my to do list to try to clean that up and find the source. My theory is, after running a longer time under load, it got hot enough to burn the oil off the surface, causing the smoke. That explains why it didn't smoke after lunch, or going back to dock, or the other times we've run for shorter periods with no problems.
To test, I was going to clean the engine surface of that stuff, then run again for 90 minutes under load and see if it returns, and if it does, try to catch it in the act and find the source.
Is my theory crazy? My test plan crazy? Other thoughts?