[Bob Bidonde]

David Renner gave me an education as to why the stalling happens. David's explanation makes fluid mechanic's sense to me. So here it is:
What actually happens when the engine stalls while stopping? When you come to a stop, your carburetor transitions to the idle circuit from the high-speed circuit. The throttle closes but stays open enough so the engine’s vacuum pulls air past it along with gasoline coming out of the idle aperture hole in the throat of the carburetor. When returning to an idle, the air to gasoline mixture is very sensitive because the amount of gasoline being used is very little. If you have the slightest bit of too much air in the mixture from a vacuum leak, the amount of gasoline in the mixture will be insufficient to prevent the engine from stalling. If the idle circuit gasoline supply is restricted or the engine lacks enough vacuum to pull gasoline from the idle circuit, the engine will stall.
The most likely reason is a vacuum leak. However, there other issues affecting the ratio of gasoline to air that will cause stalling including: An undersized or plugged idle jet; An undersized compensator jet starves gasoline flow to into the idle well; The wrong configuration of secondary idle well that causes the well to starve for gasoline.