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All Ducati v twins will flow more exhaust out of one pipe more than the other due to the firing cycles being so close together. Due to the balance pipe in the system the flow started by the first cyl. to fire[bevels being the vertical cyl.] pulls the majority of the horiz. exhaust gasses out the same pipe after that cyl. fires.As the revs go up the flow equalizes because the volume of gas exceeds the flow capacity of the one pipe. This is evident on the belt bikes but it is the right pipe that seems to flow more than the left because the horiz. cyl fires first. A bit confusing but it makes sense when you think about it.

Thanks wdietz186
very good point ! Now I strongly suspect your interpretation is the correct one:
Too lean mixture from the 40 Dell'ortos for a cold engine in cold weather and steam due to a very humid and foggy weather, following the prevalent flow of the left exhaust.
In these days I'm looking around at cars exhausts in the traffic, and a lot of them show a large steam plume sorting from there, I should have known!
Nothing to worry then! I'll keep on riding
Ciao
Giovanni