First of all - apologies to the list as single Weber carb is not 
     really a Vanagon issue (unless you build a "penthouse" atop the engine 
     cover).  But since the conversation has continued, let's finish it off 
     to avoid leaving any interested lurker in mid-sentence.
     
     Malcom brought up a lot of good points relating to icing on the single 
     progressive Weber setup for Type IV engines which I had frankly 
     forgot.
     
     I had my Weber-equipped beast in Fairbanks Alaska where it is 
     decidedly cold in winter (-63 was the record while I was there), but 
     thankfully dry.  Still, icing was a problem to be dealt with.
     
     I don't remember the exact details, but I do recall that I had a piece 
     of front tin that had a plastic fixture for a pre-heater hose.  It 
     mated up to a duct on the front cylinder tin and allowed one to get 
     some warm air off of the #1 cylinder.  Whether this was a remnant from 
     the original F.I. system (engine was originally in a 411), or i got it 
     from a bus application, I don't remember.  I rigged some sort of 
     "box-in-a-box" air cleaner to use some of this pre-warmed air in 
     winter.
     
     I also used lengths of that slit foam water pipe insulation over the 
     straight part of the manifold runners - seemed to help.
     
     'Course, under extreme conditions of temperature and/or humidity, 
     adequate warming-up before heading out also helps keep the icing under 
     control.
     
     -Blaine