A very good friend of mine rescently replaced his vacuum secondary carb with a new Holley 750 HP double pumper. We worked on tuning it for 2 days total now and no matter what we do it pops threw the carb and occasional backfires from the exhaust. I ritchened the Idle mixture screws 1/8 turn at a time till almost 3 turns out, Timing is good at 34* but it still has a miss. I'm getting very frustrated and need some advise. Car ran perfect with other carb on it, why he changed it is beyond me but we can't even put the old one back on cause he sold it. Please Help!