Hi Michael,
think it would be best to clarify first, if it's really the clutch that drags and keeps the engine on higher revs, or if it is a carb problem.
At cold weather, the cold oil might stick a little bit more than at higher temperatures, but that should only affect the first minutes. After that time, a mechanically sound clutch will not drag in a way, that the engine revs stay high.
So, if the clutch really does not free completely, you would be able to find this effect by kicking the starter while clutch is operated, if the engine then still is turned by kicking, or at least the kickstarter has more resistance than it normally has with open clutch, there might be a clutch problem.
The problem you describe may also be a carb problem. If the idle system is a little bit too lean (cold weather > more oxygene > leaner mixture), it may have the effect, that the revs come down very slowly (had this problem with my Norton recently, its no fun when you close the throttle just before of a bend, but the revs stay high
). Also, lean mixture might be an effect of leak air, e.g. at the shaft seals or at the carb flange.
Andreas
