Those Holley 2 bbls are notorious for having warped air horns that cause idling problems and part throttle stumbles or pass outs. They can't be fixed with a carb kit. If yours is showing those symptoms, I would have it rebuilt by RD Autoline out of Canada and remind them with a note in the return box that you would like that flatness of the air horn checked and flattened as necessary. This has come up numerous times on this site.