The above post can more easily be understood by looking at the contents of the carburetor rebuilding kits.
In the 1930's up through the late 40's~early 50's Carter (other companies had similar offering) rebuilding kits contained the correct gaskets, the correct fuel valve, the correct pump, the idle screw(s), the main jet(s), the metering rod(s), check valves, vacuum piston spring, and throttle plate screws.
CARTER OFFERED THREE DIFFERENT KITS FOR EACH CARBURETOR!!! That statement is correct. Carter offered the "standard" kit (sea level to 4000 feet), a "one size lean" kit (4000~6000 feet), and a "two size lean" kit (above 6000 feet).
In the mid-1950's, the "900 Zip" series of kit was introduced. This kit had the correct gaskets, the correct "base" pump (without the correct delayer spring), and the correct fuel valve.
The 900 Zip series was superceded by the 902 Zip series which contained the correct fuel valve, the correct base pump, and every possible gasket which could be used with this combination of fuel valve and pump; the customer then had to pick the proper gasket.
During this same period, many of the cheaper aftermarket kits went even further, producing often only one orifice fuel valve of a type. Example, Carter specified a spring loaded fuel valve for many of their carburetors that might see off-road use or marine use. Few of the aftermarket manufacturers make this more expensive fuel valve.
All of this is quite easily proved. And in the case of the carburetors, look at the 964, 965, 966, 967 series: one service carburetor per engine size AND transmission type.
The more specific the fit, the better the results. (my opinion, others may differ).
Jon.
