I've noticed that the low end needle adjustment screw lengthens or shortens the 'sleeve' when you look down inside the carb venturi.

I've also noticed that some engines (TT Pro .46) are tuned nicely when the sleeve does not cover the spraybar, while my GMS .47 looks like the spraybar continues all the way across the venturi opening..

Are these 2 different types of carbs?
How does that sleeve work when the spraybar is completely covered by it?
Is there a good website with the mechanics of this explained?

Thanks.
Dave