Dwayne Porter ran a performer series in a dyno test on his stocker 383 , it's in the tech archive , it was a turd , he swapped on the carter version of the same carb and it ran so much better , Ebrock has the carb made to their specs and it would appear that it is a different internal calibration than the carter version.
Two words- Strip Kit.
No calibration is perfect for all engines, if you think the carb is a "sealed unit" then you might be the problem. This is true with any carb, Holley hides its shortcomings by running rich to cover the bad spots.
Carters are very easy to setup, after reading the book on Carters and how they operate it was very easy to dive in and make changes to suit my needs and have a carb with razor sharp throttle response, good mpg with excellent WOT. Just change one circuit at a time and when its perfect move onto the next.
