I like Jerry's use of "a piston engine is an air pump".
A carburetor will not add one molecule of gasoline into an engine until it is sucked in by a piston going down while both valves are closed (it is an air pump after all).
That piston going down pulls a very specific amount of vacuum. The stock carburetor is designed to allow a very specific amount of air/fuel mixture to be sucked through it from that specific vacuum, resulting in optimum combustion.
Being armed with this information, how anyone would expect that adding a carburetor with specifications different than stock (larger or smaller CFM) on a stock engine would improve overall performance is beyond me (emphasis on overall).
Bigger CFM carburetion is for significantly modified engines.
