Holley Carb Jetting Help, Lousy Fuel Mileage

All your numbers look great. The #66 jet is a as delivered primary on just about all 4 digit list # 600's. It's hard to jet down Holley's more then 1 number in most applications. They just don't like going that lean. Not sure it would make that much difference. I agree that mileage sucks though. I'll think about it and see if i come up with anything.