69 mustang idling fast

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?

HemiRick said:

You gotta vacuum leak somewhere, if the air isn't coming thru the carb its getting in somehow. Normally a vacuum leak of this size would cause it to stall at idle, but your carb is so rich (too big jets or high float level), its not going lean enough to stall.

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Thanks! I'll check for that first.