I would definately guess an air leak. spray some wd-40 or starting fluid all over the carb boot/reed block while the bike is running. If the rpms go down, it's an air leak.
Make sure when you twist the throttle you can hear and feel the slide going all the way up and down. A pinched cable may be your problem, but I doubt your slide is in backwards. Usually when the slide is in backwards it will run near its peak rpms, rather than the 4-5k that yours runs at.