what does this little screw do?

[CasaJones]

...the little one in the middle of the picture.   It's right below the brass circle thingy.

I've checked the parts fiche and read the books and scoured the forums and I can't figure it out.   I'm sure it is somewhere I didn't look or know to look.   

Thanks

RJ

[QBS]

Can't say with absolute certainty, but if that's the screw that holds the buttlefly shaft in place, I wouldn't mess with it.

[supervision]

 That is a set screw, It stakes in place the air horn, in the throat of the carb. I also see that you have an 83 carb, I see the plug that covers access to the adjuster screw. That is a very unusual 90 degree turn in the way it moves the low speed needle. If you drill a small hole and pry out the plug, gaining the ability to clean and also later adjust the low speed mixture. Be carefull their is a small ball and spring as well as the needle itself.

[Lucky]

you have a 'virginal' set of 83 carbs. rare unobtanium....

first they didn't come on any 82. 
2nd they won't fit on an 82 without modding the upper engine mount.
3rd you'll have to deal with the return fuel line somehow.
4th, you'll need to mod your airbox, or aquire an 83 airbox to fit
5th, be VERY CAREFUL not to break the float or float support in those carbs!!
6th, you'll have to pull the brass plug to clean out those pilot screws. PLEASE count the number of turns in till lightly seated so we know what factory setting was on those carbs..that setting wasn't published in the U.S. and i only have that info on 2 other 83 carbs...

[supervision]

 Those float pins are hard for sure, Lucky. I made something to press them and it was difficult to keep the force from transferring to the posts, and breaking them. My problem is one of the low speed needles is crusty and almost broke getting it out of the body.. Other than that, mine are almost ready to install.

[CasaJones]

So far my floats and pins are in good shape.  I haven't managed to bugger them up...yet.  It's only Thursday.  Saturday is when stuff gets broke

[Lucky]

Casa, you might find this helpful for visualizing what's behind the brass plug...

http://www.xz550.com/83mix.html