'79 C10: bad cold start, perfect hot start

[twincam1449]

'79 C10. All stock. 350ci V8 with Quadrajet carb.
Stock choke with spiral element.

Warm or hot the engine starts without hesitation within a second.

Cold I have to pump the throttle to use the accelleration pump and start for a couple of seconds.
Choke valve is closed when cold and fully opened when warm/hot.

If it has run for a a minute or so, it starts without problems.
Even after a couple hours it starts fine.

Do you think it's the choke, or could fuel drain back to the fueltank? Which makes it hard to start, as the pump has to pump it back to the carb?