I've got a 69 mustang with a 302 rebuilt ATK engine. It's idling extremely fast (probably around 2,000 rpm). The idle speed screw and idle mixture screws are leaned out as much as possible but it doesn't seem to have an effect on the idle speed. I will also get intermittent backfires when the throttle opens up. I pulled the spark plugs and it's definitely running rich but I can't lean out the carb anymore. Any tips?