I'll butt in here.
It was Marc's insight from a known problem he found. He noticed that even when jetted better, his little Pinto 140 cubic inch carb on his 250 six felt like it had even less power than his stock 1-bbl Holley when extended.
I quickly remembered from David Vizards work that with the Holley Weber 5200 carb on even a 121 cube Pinto engine, it could benefit from 31 and 32 mm venturi size, up from the stock 26 and 27 mm respectively. Stock, the carb was limited to about 100 hp or so. When upsized internally, the 31/32 combo was good for 130 hp or so. There is enough meat in thecasting to go up to that level.
Australian 200 and 250's had 120 to 130 hp net. They used a Weber 34 mm carb with two 30 mm venturi's.
Mathematically, there is a critical point when carb venturi area is too small in relation to the cubic inches of the engine. At this point, the air speed at wide open throttle goes up too much, and the engine feels like a vacuum cleaner with a blockage. Engine dyno operators tend to find that over 265 feet per second at wide open throttle, the result is a loss of performance and power.
General mathematic rule is take the total cubic inches of the engine, and then divide that number by the total venturi area in inches with all throttle wide open. A stock 5200 Weber is about 1.71 sq inches, and on a 250, it has got over 145 cubes of engine for every square inch of venturi, and the air speed gets very high when the throttle is opened above 3000 rpm on these engines.
From observation from the Ford, GM and Chrysler six cylinder kingdom, if the cube to venturi figure is greater than 125, then the carb most certainly can be reamed out and you'll get an improvement in peak power. Depending on the casting quality, most Holley carb bodies can take a 62.5 thou ream out, or up to 125 thou if you repair any porous areas with Deveon/JB weld. On little pip squeek engines like 67 cube Healy Sprites and even 121 cub Pinto's, just opening up the venturis can make a 25% increase in power if other mods are done in unison.
For the 5200 Holley Weber, the limit of the increase is that the throttle size should never be more than 1.0625 times the venturi diameter, and best results occur when the stock throttle size is 1.125 times the venturi diameter.