Something puzzle me so I need your help and advice

[mrclubike]

From what I can understand from your description of your problem Mine does this also.
It really is only noticeable in the winter time.
I am fairly certain mine is leaning out I checked the  needle and it is already in the top position.
So I just assumed it was carb icing or just the nature of it under cold conditions.  

[Barry]

There would be some natural leaning out during the winter due to the increased air density at low temperatures. A temperature drop from 25 Deg C to 0 Deg C  results in an almost 10% increase in air density and a proportional reduction in mixture strength. The effects felt in top speed might be made even worse by a 3% lower calorific value of winter blend fuel and a significant increase in aerodynamic drag. CV carbs should compensate to some extent as this is just the opposite of having to cope with higher elevations but in the end top speed has to fall.

I remember back in the early 70's bikes always seemed to run better in the cool of the evening. I believe that was because they were set up rich enough for the increased air density to make more power.  Where as later bikes set up lean for emissions targets would run too weak in those conditions.

[georgesgiralt]

So, this afternoon I went for a ride. Tank cap loose (the ratcheting does not work very often, so i have done a hole under the handle to put a nail on it when I want to open the tank...)
I have to admit, this time the Guru was right. With the tank well vented, the problem is almost gone (the bike always had a bad spot around 4000 RPM)  
As I've checked quite everything, she is very sweet indeed. I could not use her as I planned because we where approaching rush hours and the freeway was beginning to get packed.
But tomorrow morning I'll see what's in her guts !  
I'll think I'll buy her a new tank tap ...  if only I knew how to remove the lock from the present tank cap ....

[Bob_Roller]

I wonder if the internal parts of the fuel cap would be resistant to carb cleaner

Most likely its varnished fuel deposits inside of the fuel cap causing the problem .

[georgesgiralt]

Bob,
The ratcheting is done, the air venting is done, the appearance is sh...
So I think it is time for a new one. (given the bike looks like new with a lot of parts freshly painted or powder coated).
I will dismantle the old one when I get the money to buy a new one and see what is the cause for the vent sticking when cold and of course get the lock to install in the new cap  8-)