Carb Help Needed
I would start with the basics.
•Float levels.
•Idle screws (bottom them out then back out 1 1/4 turn. Some say 1 1/2 turns).
•Hit every air bleed with carb cleaner.
Those are the first 3 things I do if a Holley is acting up.
If those 3 things don't work……
•Pull the bowls
•Hit every hole with carb cleaner
•Replace the power valve (because you are in there, may as well)
•Pull the needle and seat out of the bowls and hit with carb cleaner.
By this point every Holley I have had is running smooth.
•You may also want to check (replace) the secondary diaphragm and make sure the check ball is in place.
•Get one of these
http://www.holley.com/20-59.asp
I would also reset the stock jetting.
At 6 mpg you are getting 1/2 to 1/3 what you should be getting.