from what i have seen when one of these engines gets some hours on them the blow-by in the valve cover gets high and oil contaminates the pcv valve in the valve cover. Even though the valve cover does have a baffle in it the oil still vaporizes and gets past the baffle.
During Pm's cleaning that pcv valve is imperative. If that pcv valve sticks open it will allow oil vapor (which builds up over time) to go through the hose to the mixer/governor and that's where the smoke comes in.
At least that's my take on what i think is causing the smoke.
Using the wrong engine oil or not servicing the engine 'on time' consistently will contribute to premature piston ring and valve guide wear creating the blow-by.

and of course i could be totally wrong about this too, it just seems more logical than anything else i can think of after seeing it quite a few times myself too :o)