There is a little piston in there with an oring on it, when you rev it,it pushes a little squirt of fuel straight in to the vortex of the carb, the oring wears out and let's air in when it shouldn't that's why you can't adjust it and nothing has the desired effect with screws.
The reason sthil have done this is due to emissions, the saws are running as weak as possible and need this to accelerate fast.
If you look at the back of the carb there is a oval little inverted cap,behind this is a canal to the piston, if you use a blunt punch and tap the cover into the canals without puncturing it , it will block this off, solving the problem, reduced acceleration is barley noticeable, especially if you have been tortured by this problem!
Saw going bluuurr bluuur when you rev it , not idling properly, obviously clean carb properly first may just have muck in it!
Hope this helps someone
Sharpster, December 2013