Too lean.  If you want factory settings, don't remove the brass plug.

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Remove the brass plug and you let the genie out of the bottle.  Then you have to fiddle with it all the time, because you can.  And it makes a difference when you do, things get better.  And then worse. And then better.

Then you start keeping mileage logs and fiddle by quarter turns and keeping characteristic logs for each setting.

Then you realize that it changes by season too.   And altitude.

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The screw should turn easily.  If not, it is corroded in place with brass oxidation and you need to hit it with a penetrant spray like Kroil or WD40 before attempting to force it.

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Don't turn it out more than 3 turns or it can vibrate out and fall out when riding.  There is a spring behind it that gives it retaining friction.

I have a question about this also. I've noticed some strangeness while messing with this air mix screw. I bought my bike (06) used (I'm the 3rd owner).
First off, the PO told me he'd done no mods on the bike and I believe him. The brass cover was already off so I'm assuming the 1st owner did that.
When I turned the screw CW to determine where my starting point was it was only out 1 1/4 turns. I turned it CCW 1/2 turn past that (so it was now out 1 3/4 turns). Adjusted the idle screw to bring down the rpm's too.
It starting backfiring like crazy when I shut the thottle down fast...like really loud gunfire backfire! It hadn't done that before??? Just a gentle "poof" when shutting down previously. After the adjustment, it didn't "poof" anymore at shutdown but man did it backfire while riding!  I set it back where it was and it seems to stop the backfire and return the shut down to a "poof".
I'm thinking that because the air mix screw really isn't out very far that maybe the 1st owner had rejetted. Does this seem like a logical explanation?
I also live a mile up 5280ft...
Any insights?

Thanks verslagen, let me see if I'm getting this right. Since I'm a mile up the idle mix screw won't need to be turned out so many times to richen the carb? I was under the impression that if I richen it, it would stop backfires not cause them.
Right now it runs good (no backfires just a little poof on shutdown) but I plan to put on (maybe) a Dyna over the winter.
I'm stock on the air filter and exhaust.
What I'm really trying to do is keep it running as cool as I can.
Thanks again

is this in the carb? or the idle screw?

we left the needle alone when we rejetted, is what's being discussed in the carb? brass cover?

Well, since we don't have a temp gauge I'm kind of wingin it here but essentially I know the bikes from the factory run lean i.e. hot. So what I'm hoping to do is get the temps down so I don't shorten the life of my spiffy new bike...run as rich as I can w/o backfires or a drop in performance.
Like I said, it runs good right now just want to make sure I'm not runnin over hot.

i think i saw picture of before and after in thech section under toc where it show how to do that

Our bikes are jetted to pass emission standards at sea level, where they are delivered...  that makes them a little lean at sea level, but at 5000ft you may actually be a bit rich, or likely close to correct, with stock jetting.

Lean backfire is a common problem, because most populations are at sea level.  
You can get a rich backfire too...
Lean backfires happen when there's not enough fuel in the cylinder to make a good boom, and unspent fuel goes past the combustion chamber into the pipe.
Rich backfires have too much fuel and not enough oxygen for a good boom, and again, unspent fuel goes into the pipe and pops there...

A good indicator of overall jetting is the weather...if popping/backfiring increases on humid days, or at higher elevation, that indicates rich....less popping at those times indicates lean.

Also,.. too low of an idle speed will cause backfire on decel... you might just try raising your idle speed by 100rpm or so, and see if that helps...