Here's a snippet from this guy's site:
http://www.bob2000.com/carb.htm
Double Pumpers and Gas Mileage (or lack thereof...)
Have you heard people complain about the gas mileage they get with performance-type double pumper carbs? There is a reason that the 0-4776 through 0-4781 double pumpers get bad gas mileage. It's the jets! Surprised? Holley sizes the jets and air bleeds on these carbs so that they run on the rich side at cruise speeds. They make more power this way, at the detriment of gas mileage. These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations.
The way to do this scientifically is to measure the diameters of the openings of stock jets and PVCR's using drill bit diameters. Then calculate the total area of all the openings, and add them up.
A = 3.1415 x dia x dia / 4
Decrease the main jets for proper cruise mixture, and enlarge the PVCR's until you get back to the original area of all the openings. This way, your cruise mixture will give you good gas mileage, and under power, the engine will have proper mixture.
Cheers
Steve
