Have you disconnected the cable from the throttle bracket, see if it's the cable sticking or the carb itself? I know it said replaced but that would indicate not ruled out. Could be a slightly bent butterfly shaft too, or warped carb base which has the same effect and both of which need attention form a decent carb guy who knows q-jets.
Check if it's the original carb too, they are the better ones for 403 anyway. 17059253 (most common) /17059223/17059553 or same with 9 changed to 7 or 8 for different years.