Update:
Chuck was right!
Found a vacuum leak today, the throttle shaft has some play. Spraying brake cleaner onto the shaft helped, the engine ran better but still not at all nice.
After taking the carb apart I found that the float chamber was surprisingly clean, but the entire intake area was quite dirty. Also the main jet was completely clogged from what seemed to be gummed up gas, the cap jet was basically the same.
So I took the carb off and disassembled it. To my surprise, it even came apart without much of a fight.
I cleaned everything up, polished the venturi because the inner surface was quite rough (it's a die cast venturi), changed the gaskets for new ones and put the carb back into the car.
What a change! The engine runs a lot better now, not perfect yet, but certainly way better than yesterday! It can even move on it's own now!
Over the winter I'll install new throttle shaft bushings along with a new shaft and check the idle circuit (which runs fine atm) and the seat of the GAV-valve.
Thanks a lot everyone!
