Loosen the clamps on the carb rubber boots after removing the air cleaner assembly.
Gently turn the the carb where the throttle cable entering the slide cap is facing more toward you.
Unscrew the carb slide cap and gently pull the slide out.
Check for hardened cosmoline grease on the slide - clean it off with kerosene.
Have a buddy pull on the slide and holding the cap, while you work the throttle, and check for any cable binding in the full range of mechanical travel.
If binding or roughness occurs, take off the cable at the slide cap and check for casting flash right in that area. Will need a tiny model makers file to remove any casting imperfections. Chances are this is where the binding will occur.
The exact same problem I had on my bike. After fiddling with it for a long time - my dealer found the casting flash which slipped by me...
Regards, Foggy