How much carburetor area is too much (viz., no more improvements)?
When the WOT vacuum is just below the rated vacuum: 1.5" of Hg for 4 bbl. carbs and 3.0" for 1 & 2 bbl. carbs.
If the carb is too big (or engine too small) the carburetor will not deliver its full rated CFM.
The reverse is also true: if the carb is too small (or engine too big) the carburetor will deliver more than its rated CFM.
What else to watch for: the fuel curve (air bleed, emulsion tube etc.) for almost all carbs is based on it operating in its designed WOT vacuum range. If you add too much 1 or 2 bbl. carbs so that the WOT vacuum falls below their rated 3.0" the air correction will be out of range and run lean at high RPM. The reverse (Holley 390 on a 454) will run rich.
