The Stovebolt Site Forums The Shop Area Engine Shop why does vacuum go down when rpms go up?

Very small is typically 1.5" to 3" of vacuum, smaller carburetor = higher number.
Slightly more power by going below 1.5", but unless the carb is very sophisticated (Weber DCOE & IDA, Dell'Orto DHLA, Mikuni PHH, Solex, &c.) the venturi signal is too weak for accurate metering.
EFI can work as far down as .25": really big TB.