The parts diagram for the NAA carb shows 5 different springs:
Carburetor Main Adjust Needle Spring
Carburetor Idle Adjust Needle Spring
Carburetor Throttle Lever Adjusting Screw Spring
Carburetor Choke Shaft Spring
Choke Flapper Valve Spring
The first 2 are installed ahead of each of the needle valves, so if the needle valves are in place, it's unlikely to be either of these. The next one is on the exterior of the carb on the throttle linkage, so if it "fell out" I would guess it wasn't that one. The next one is also on the exterior of the carb, on the choke shaft, so the same probably applies about it not having "fell out". The choke flapper spring, which old already mentioned, is in the throat of the carb, so it seems to be the likely suspect.
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