[Selkirk]

The point of operation requiring the most intense spark is right at off-idle. If a car is developing ignition problems the driver will always complain that it stalls on take off.

Your carburetor actually has three holes where fuel enters; idle; off-idle; and run. If the engine can rev up normally, then all three of them have proved that they can operate correctly and make the transition from one to another

1st I'd take a vacuum gauge, attach it to the engine and set the fuel adjusting screw for maximum vacuum and leave it there. That's the proper way to adjust idle mixture. Don't try to do it by ear.

Then I'd go after the ignition system. You already visited the spark plugs, but what about the points, distributor, and plug wires? Somehow, some way you are losing ignition energy just when you need it the very most.