First, make sure that you have properly identified which is the idle "speed" and which is the idle "mixture" adjustment.
The best way to positively identify the idle speed screw is to take it all the way out and look at the tip. If it is pointy, it is the mixture needle. If it is blunt, it is the idle speed. The idle speed may have a lock nut on it that you will need to loosen in order to turn it easily, and then tighten to have it stay in that spot.
The next thing you should do is to make sure that the throttle cable is not too tight and holding the throttle open. If you twist the throttle open, then slowly let it return you should feel the tension come off the twist grip while it still has a tiny bit of rotation left. If there is spring tension all the way down to the twist grip stop then you need more slack in the cable.
If you don't have enough slack in the cable it might be because the cable is not routed properly and it is pulling the jacket out of the carburetor or twist grip a small amount. With the bike running you should be able to flex and move the throttle cable without it changing the engine RPM.
Many of the throttle cables have a mid-cable adjustment for the jacket length. This will be a little metal fitting where one piece threads into the other. Loosen the lock nut, thread the fitting together a bit more, tighten the lock nut again. This will make the jacket shorter, which will loosen the cable inside.
If you have plenty of cable slack then the throttle slide should come down and rest against the blunt end of the idle speed screw. If you just back the screw out the slide should drop and the idle RPM drop with it. If the slide won't reliably drop then it is an indication that the carburetor is either damaged or just so dirty that the slide won't move freely.
Rod