I had a similar problem, though mine would not run at all. My issue was the seat for the valve in the top of the vacuum tank where the line from the manifold connects. The seat had become loose in the top housing, so even when the vacuum tank became full of fuel and the float rose and the valve closed against the seat, it could still draw vacuum round the loose seat in the housing. Thus it never sealed and just sucked fuel directly into the manifold, providing a way to rich mixture for it to run.

Took a while to find this. I pulled way to much other stuff apart looking for the problem.

I fixed mine by glueing the seat back into the housing with some medium strength threadlocker. It is fuel resistant.

Had no further problems since it was repaired this way.

Where Are You From? Down under

Do You own a car built by Durant? D40