Hmmmmm, interesting. This is what I would do, the next time you do the cold start, right after it starts to run then quits, look into the venturi and operate the throttle a couple of times to make sure you still have fuel available. I'm still thinking it's a fuel problem, you could have an internal leak in the carb or a weak diaghram in the fuel pump that's allowing the float bowl and/or the fuel line to empty between starts. The fact that it starts to run, and then after a few seconds of cranking will run, seems to rule out an ignition problem.
