[davepl]

I'll assume the timing is off, but if it comes back as correct, here's a theory:

You've got a vacuum leak. This causes two problems, a lean mixture from and a weak vacuum signal to the carb.

When the engine is cold, the choke is on. This makes the mixture MUCH richer, hiding the vacuum leak. It also "sucks" on the jets harder with the choke door closed, so those internal vacuum systems get a stronger signal.

Then the engine warms up and the choke comes off and the vacuum leak manifests itself.

Can I prove it? Nope, but it's my best guess from across the Internet.