Doesn't like the cold!

[BlackDogZulu]

3TB, 1994, 26k miles.

Minor issue, but would appreciate your views. Starting point: bike is starting and running well. In fact, it's never been better. Pull the choke, engine starts within 2-3 turns, good fast idle, choke off within half a mile and then runs and idles correctly. Ran fine all the way through last winter, too.

The last couple of mornings have been below zero, perhaps -3C. It's been a lot harder to start, firing 3-4 times and dying before getting going with the throttle cracked quarter of a turn and held there. That's not too bad, but once it's properly warm (after 10+ miles of 60mph plus) it's trying to die if I back off the throttle (e.g. changing down for a roundabout). This morning was warmer, and the bike started and ran fine.

It's not a big deal, especially as summer is coming, but it looks as though something has deteriorated since last winter. It feels a bit like carb icing, but I didn't think that was an issue with these bikes. Any ideas before I start tearing it apart?