Some Rochester 2GC carbs use two balls, others one only. Would be easier to give an intelligent answer if the identification number of the carburetor were given.
However, since that is not present, "shotgunning" the answer:
All of the 2GC series carbs that I have personally worked on had the larger steel ball under the venturi cluster.
Those that I have worked on that have a ball under the pump use the smaller aluminum ball under the pump.
It is possible that a siphoning action could occur if the smaller aluminum ball were placed under the venturi cluster, thus artificially enrichening the mixture causing the engine to stall from an overrich mixture.
Jon.
