They are sometimes pressed in, sometimes threaded as far as I have seen.I believe that spring loaded bearing is just there to put location pressure on the choke shaft so it stays where it's put.
I think the nozzle check valve is the component below the number 5 in that diagram. Often it is under a welch plug but that doesn't appear to be the case here. These valves can be tested & sometimes even repaired or replaced. If the issue is definitely over fueling I'd be looking at the metering & adjustments first. Have you tested for carb leaks & pop-off pressure?