Race car mystery-Help us solve PLEASE

we also replace the intake gasket and have since tested it with the ol carb cleaner test and no rpm increase,I agree on the valve spring, losing 1.2 to 1.5 sec is a big loss,we also changed carbs and it ran the same-we have a/f meter also and cars runs 12.0 to 12.5 at W.O.T. --Steve