Yes, stove pipe thing shouldn't make any difference in VIC temperatures. But, dunno about sub zero alpine temperatures though, which can depend on your carby condition and tuning. Mine doesn't have the stove pipe or the shroud at all. Drives just ok in high altitude, but fairly gutless. I think the moist air doesn't help and even though the cold air does help for more boost in theory. The solution is to make the air/fuel mixture rich, which I'm not too sure the job of the auto choke.
It's not too hard to find out whether the auto choke is working ok, but an intermittent problem can be pretty tricky to locate.
Sticky throttle shafts can be intermittent issue, or may not show during your typical throttle test.
You can also try what I suggested above, which is adjusting the choke lean/rich setting.
I would first give the carby a good carby cleaner spray clean while revving high.
IF that doesn't help, pull the carby out and tighten the three bolts that holds the throttle body to the float.
Since the gaskets aren't available on their own, you might have to cut the gaskets using gasket paper or spend $78 and get a rebuild kit.
If you do rebuild it, take time and do it properly. Plenty of carby cleaner to clean the interior passages and plenty of high pressure air to clean the gunk.
Be careful with the plunger boot, as it can break easily. Don't loose any springs or balls inside the float, 'cos springs aren't included in the typical rebuild kit.
