Hard cold starting carb 502 (Choke problem?)
 

I'm helping out on a couple Gen V carbed 502 with the Weber/Carter AFB type carbs. Both engines act the same way.
It's very difficult to start the engines after sitting for a few days. The bimetal spring, manifold mounted chokes are typically closed at startup and the vacuum diaphram pulls them open as soon as the engine starts.
There's plenty of gas in the bowls. They'll sqirt the pump jets at least a half dozen times after sitting.
I recently rebuilt these, so they're clean and adjusted correctly.
Spark is good, wires and plugs are new. I'm reading nearly 20 kV on startup cranking.
I've tried adjusting the chokes from closed at cold startup to about 1/4 and 1/2 open. Still hard to start.
The only way to get it to fire immediately is to manually hold the choke closed then pull it open while cranking. Then it fires up right away.
I've always been able to get the AFB clone carbs to run as good as fuel injection. What am I missing here?

do they attempt to start and stall or not fire at all ?

No fire at all, just a lot of cranking, but there is spark. Once they start, they're fine.
On other carbs in this hot Ozark weather, I've set the chokes at about 1/4" open. To start, all they take is 1/2 throttle pump, then fire immediately and run smooth.

My merc carter carb was the same way. Always had to pump the heck out of it if it sat for more than a day. I solved my issue with a Holley :)

that is normal on the carters, pump the he11 out of them when cold.

I had the same issue with my 502 with same weber carb. I solved the problem by rebuilding the carb. I also replaced the accelerator pump while rebuilding, which I think was the real issue.