It's not a mix screw, all that screw does is lift the barrel.
To start, put the screw all the way in, and give it 2-3 full turns out. This is to zero it out - that's actually the LAST place you want to adjust when setting your idle.
On top of the carb, there is the cable tension nut. Loosen or tighten that to about halfway.
Now, on the throttle grip, set the cable tension to remove all of the slack. You may have to play with the cable tension nut on top of the carb at the same time to get it set right.
You'll notice you're ready to start when you feel the throttle cable nice and taut. Now you can fire it up, and adjust the fine tune idle screw on the carb to where you'd like the rpms to be!
The butterfly you're referring to is for air fuel mix, you'll want it heavier than comfortable for break in period, then you can really start dialing in your carb and leaning it out.