In the 1960s, intake manifold technology was very primitive and Webers made more power than a 4-barrel intake system, primarily because they don’t interact and therefore allow equal mixtures to each cylinder without one disrupting flow to the others. They also had the advantage of being much more adjustable for tuning purposes.
Webers do, and have always suffered from reversion issues. When that pressure pulse happens at intake valve opening, it has no place else to go but back up through the carburetor. The carb doesn’t know or care which direction air is moving, if there is flow, it meters fuel. (This is the place that fuel injection has its one big advantage). In an SAE technical paper, Ford engineers estimated that the 1963 Indy winning engine lost 3 to 4 gallons of fuel pumped out of the carb mouths and lost to the atmosphere during the course of the race.
By the nature of an IR type system Webers really haven’t evolved or gotten any better. It is what it is, and other than changing the length of the intake runners to change the resonant frequency, there isn’t a lot you can do with it.
There has been a huge improvement in 4-barrel manifold technology in that same period of time. Today, you can expect roughly equal power from a 4-barrel carb, Webers, or fuel injection. There will be some places in the RPM band where one or the other will make a little better power because of tuning, but overall, across the entire RPM band they will be similar. Also with newer carbs like the Demon RS carbs from Barry Grant, you now have just as much tuning and configurability as Webers.
The bottom line is that Mark was quite correct. Today Webers are strictly for looks, and for that matter, Fuel injection is not a performance improvement either although it does have advantages in cold starting and a few other drivability areas.
By the way, if you really want to know about this in detail, try to lure Adam into this discussion.
Kevin