[PeteF]

I'm surprised the idle/mixture screws have any effect, if you are "idling" at 2000 rpm, well above normal idle. Have you verified that both throttle plates are fully closing (looking down the bores with engine off), and that they actuate smoothly throughout the range. Check this with return springs removed, engine off. Also verify that the backfires didn't blow any rubber caps off the carb or manifold (if you have any closed/unused ports). That could cause an air leak which could raise idle rpm. I think there is a Holley troubleshooting section on their website that might have more advice.