The ultimate carburetor is fuel injection.......
No trying to be smart about it, but in reality, a carburetor is nothing more than a crude mixing device, subject to so many outside influences, that it is hard to get a one-size-fits-all carburetor.
Intake tract design is a huge factor first of all. The length, and size of the runners has a very big effect on mixture and velocity of what enters the engine.
Intake air temperature (ideally 140-160 degrees F) is very hard to maintain in an older thermostatic air cleaner system to promote the best combustion possible. Also keeping the intake temperature consistent is a must. That was done crudely for many years with heat risers and coolant circulation. Even EFE electrically heated grids were used too, but they proved to burn out in time.
Several smaller venturis are better than one large one, because the air flow is more evenly introduced and more efficiently atomizes the fuel to mix with incoming air. Ideally a 14.7 to 1 ratio of air-to-fuel is sought after, but next to impossible to maintain with a carburetor.
Carburetors are also subject to one factor that cannot be controlled: wear. Throttle shaft wear, and other moving parts make maintaining that 14.7 to 1 even harder. Couple that with engines of poor design to start with (i.e. large intake valves, huge camshaft durations and overlap) and they don't respond well no matter the good intentions to make them better.
Consider this as well: The last car to use a carburetor in production was a 1989 Hyundai Excel. The last truck was a 1993 Isuzu pickup.
Let's just say they have their limits. Efficiency is not one thing they were ever known for.
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