The carburetor is fine for a 305 so that’s not the issue.
I assume you mean the engine is stumbling bad and/or trying to die upon cold startup?
The reason why could be due to the fact that the carburetor does NOT have a choke pull-off diaphragm. Unfortunately, that’s the way they’re made so it takes some tinkering to make them tolerable. I can’t speak as to how you’ve done various adjustments.
The choke PO is designed to open the choke flap about 1/8" immediately upon cold startup and from that point it opens gradually as the choke heater warms up. Without the choke PO it basically overchokes itself.