I can't figure out what's happening with my sbc 350. It idles smoothly and revs fine in park. Under load it backfires through the intake. It has ported 882 heads that are freshly decked with new valve guides, valves and springs, new cam and lifters and new intake with a Holley 675 carb. I checked for a flat cam lobe, vacuum leaks and made sure the firing order is right. Can't figure it out. All I know is when I adjust the mixture screws the rpm doesn't change until the screws are all the way in but I have them out about 2 ¹/² turns.