Seems to be a misconception out there that large carburetors make an engine run richer. In reality, the engines actually run leaner if the carb is oversized. But I’ve had good luck with carbs up to 780 cfm driving around just fine with the Y engines. It’s just when the secondaries are opened that may force the issue on carb sizing but with an oversized vacuum secondary carb, air flow is diminished enough that the secondaries are either late in opening or will not open at all. It goes without saying that almost any double pumper carb will give the engine too much air when opened at too low an rpm but a vacuum secondary carb is self-compensating in this regard. With smaller primaries, air flow or velocity through the venturies is higher which in turn gives a better throttle response at low rpms at the expense of high rpm air flow. Atomization is also obviously better with the smaller venturies which in turn helps with fuel efficiency.
I’ve had most of the popular carbs on my 272 and the 525 cfm Road Demon Jr. is the clear winner. I’ve run several of the ’57 ECZ carbs (390-410 cfm) and these do not have the benefit seen from the advances that have occurred in metering and circuitry that’s employed in the newer carbs and although these older carbs could be tuned to satisfy either the economy or performance characteristics desired, would not do both simultaneously in a satisfactory manner. I did run a 500 cfm Carter for a fair amount of time and although it got good fuel economy, it was always touchy when the secondaries were first opened at the lower rpms. Part of this is due to air leakage around the weighted secondary blades and the lack of intial fuel to compensate for this. But beyond this, still a good all around carb.
As usual, just my two cents worth.
Lorena, Texas (South of Waco)