I have finally found a cold start routine that seems to work over 90% of the time for my Trabant. Including temperatures down to -10'c and with a slightly flat battery (slow cranking). Open the fuel tap. Tap it a few times. Count to 25 and pull out the choke fully while you do it. Start cranking the engine while pushing the choke in slowly. At a given point the engine catches and blipping the throttle gets it going. Push the choke in as appropriate.