Low idle issue.

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Hi all. Recently my cars idle has dropped to 700 or so from 850, but when I apply throttle and then take my foot off quickly it drops down to 500-400 for a second then back up to 700. I revved the car to 5K and released the throttle and it idled right down and died. 3 days ago I took the exhaust off the weld up a leak and decided to get rid of the cat converter, and welded a straight pipe in it