Holley with vaccume secondaries. It's not mine. A guy on another forum asked this question. I told him that it will hurt milage and then he says that it wont and I am wrong. Not sure why he asks a question and then answers it but whatever. I told him that when I switched carbs on my old Chevy truck and another car it helped milage. Sometimes people put much too big of a carb on a motor than what it really needs IMO.