Any idea what causes this?

I have a similar problem but I think it's choke related now (it's cold here and the Super 6 lemon is my DD) I'm not sure if it opens to slow or fast but if I drop it to drive before good and warm it dies. I had a loose carb gasket I didn't notice til I had the air cleaner leaning on the carb and it moved. Also when I bought the car with a holley 1920 it was exactly like yours. I got a carter 1bbl off a friend. I found that the guy that had the car put the carb gasket on backward. There is one hole on the manifold, one on the carb so wouldn't you think the hole in the gasket went between the others. I got a new one and put it on right...car ran much better. The 71 swinger i used to have had a manifold gasket leak and ran like yours.