Fran and Gord, the dual carbs on the Z-28 was an option for racing where thew engine will be modified and see over 7000 rpm.It was over carbureted on purpose for that reason.
My actual experience on 261's says adding the equivilent of an additional carb with no other changes adds about 10-15 HP above 3000 rpm with no loss in fuel mileage,that's a 10 percent increase in power.
