You shoulddefinitely adjust the idle and air mixture screws if your FA50's not running well. Take a look at the [link|http://moped2.org/forum/forum_posts.asp?TID=11658|carburetor] diagram and make sure that the air mixture screw is 1 3/4 turns, as instructed, and that the idle screw is adjusted properly.
Honestly, I wasn't able to figure out how to set the idle to "1500 +/- 150 RPM," so I just ran the bike until warm and then adjusted the screw until it "sounded right" -- whatever sounded like it should be going at.
Also, make sure that the choke cable is completely off when running the bike, and especially while adjusting the idle / air mixture screws.