Why changing Line Width caused burnt marks in the prints?

[ultimaker_user 0]

Hi, why changing the Line Width caused burnt marks in the prints? Usually happen at an edge of a circular hole in the middle of the prints? I can see some extra thin filaments being deposited at one side of the hole and some of them got burnt. 

[ahoeben 1,160]

If you increase the Line Width, what happens is that the printer extrudes more material than with the smaller line width. The additional material gets flattened out by the "shoulders" of the bottom of the nozzle tip (ie: the flat part of the bottom of the nozzle tip to the side of the nozzle hole). If you increase the Line Width too much, there some material may "curl up" beyond the shoulder and stick to the nozzle for a bit. Material that is kept at extrusion temperature for too long may carbonize ("burn"). If it does, and then gets pulled off the nozzle, you get the burnt mark.

[ultimaker_user 0]

Thank you. What happen when we reduce the Line Width? How come I also see burnt marks when reducing the Line Width to a value smaller than 0.4mm which is the size of my nozzle?

What is the best value for Line Width if I use a 0.4mm nozzle?