I've never had that problem. Is it an old pump? Is there a replaceable filter? Could it be sucking air somewhere between the tank and pump? Could the be an obstruction between the tank and pump or pump to carb? Do you have multiple carbs? Is there a lot of heat near the pump or lines? See if the GMC fuel pumps have a different part number. Some will just because the have a vacuum pump side as well as fuel. But the 270-302 may have a higher volume. Let us know how you fix it.
