Running like crap when it's cold outside

FWIW, I installed a Air/Fuel mixture gage on my 73 Duster with a Thermoquad. I live in Central Alabama, which averages 90 deg F in Summer, and about 50 F in Winter, with of course colder and hotter. In Summer, on a 95 deg F day (35C, doubt it gets that hot in Norway?) I have it set to run at about 14.7-15 air/fuel. It is now getting colder, and Sunday it was about 60 deg F (15C, your summer high, haha) and A/F was around 16. So you may have a carb that is borderline lean in the summer, and is way lean in the winter. I normally change metering rods about this time of year. At about 17 a/f, I notice a change in the engine performance.

What ignition system are you running?