Time to rebuild?

Good you determined it is an O/F problem and the engine is sound. The first responses of "just rebuild the engine" would have set you off into la-la land and still wouldn't have fixed the problem. Once warmed up, the choke should be fully open. You should feel the choke spring pulling hard. In-between is what they try to design for and could never do near as well as electronic fuel injection. Your year has all kind of add-ons like an intake air heater and choke heater, all attempts to kludge the mechanical system into working acceptably.

The biggest thing to verify is that your "choke pull-off" works. Those are often damaged by spraying carb cleaner. To test, use a Mighty-Mite hand pump on the vacuum port and see if the rubber diaphragm holds vacuum. If you don't have one, push it in, block the port w/ your finger and see if it stays in. Of course, it will extend slightly until it builds vacuum. The theory is the choke should be tightly closed when cranking to give a rich mixture. The second the engine fires and speeds up, it should pop open slightly (manual shows a drill bit measurement), due to intake vacuum. If not, the engine will die from "too rich". Wish I had understood that decades ago when I kept fussing w/ the choke adjustment in my 1969 slant between summer and winter. I don't know if most mechanics then even understood how it was supposed to work.