Assuming no fuel leaks and no vacuum leaks and a clean carb and clear, clean fuel pathway from tank to carb:
On the hobbit, drive a mile or so to warm up the bike to normal operating temperature.. This is important. If you set idle on a cold engine, it will only idle when it's cold.
With the engine hot, adjust the speed screw so the engine spins only fast enough to keep running and not die..
Now slowly back off the air-screw .. if engine RPM starts to fall, stop and turn the air screw the other way, slowly... What you want is to hear the idle speed increase. This means the fuel:air mix is becoming more correct.
Lets say you turn the air screw inwards and idle speed increases. If you keep turning, at some point idle speed will begin to drop. Find the air-screw position where idle speed is highest.
Now reduce idle speed with the idle speed screw... to the lowest possible idle speed. Again, play with the air screw until idle speed increases. And, again lower speed with the speed screw.
Take it for a ride around the block and see if it idles nice and slow and smooth when you come to a stop. If not, go through the process again.
A PA50 with it's reed valve intake is capable of a very low, smooth idle.