Difficult cold start

I am returning to this problem as it still persists. Reason is not cold climate as we have summer here in finland and temperatures have been high (25 degrees celsius).
Problem is still there. If car stays few days without start, it is very difficult get it running. Starter runs fine.

What I have tested so far:
- added one way valve to fuel line to prevent gas flow away from carburetor bowl (i don't know if that is even possible)
- checked ignition coil resistor bybass and tested with resistor shorted.
- locked choke to open position as it is not needed in warm climate.

I am wondering if it is possible that gas evaporates from carburetor bowl causing long delay to get it full again with starter? fuel pressure seems to be very low in this pump.
Bowl vent stays open what makes me wonder if it helps gas to evaporate? How it is designed like that and why is this valve needed?