I had this same problem on my '81 Westy.  It was an icy cold January day in Tucson, with the
thermometer barely poking its head above 70 F.  At first I thought the high speed dipstick had
jammed, but it turned out to be the new roller bearing muffler assembly I had just installed
the week before.  Apparently it just doesn't like the cold weather.

Bill in AZ

----- Original Message -----
From: "G.M.Bulley" <gmbulley@BULLEY-HEWLETT.COM>
>
> If all those check out (which I virtually guarantee, one of these is the
> prob) then you have a fuel system fault, like injector #5, or a clogged
> Fetzer valve, worn clutch belt, or wallowed Kuhneutson bushings.
> Sometimes the piston return springs get loose in cold weather.