How much does 1 jet size affect A/F?

The manufacturing tolerances from one jet size to another can vary enough to make enough of a difference between any two given adjacent jet sizes to negate any real effect. In other words, if you tried a dozen of the same sized jets an measured each ones effect there would be a significant range of A/F ratios for the same sized jets.

Some companies offer blueprinted jets that have a lot closer tolerances, but I believe the are for the Holley type carbs.