Normal idle is from 600-850. The first thing that comes to mind for sputtering is a vacuum leak. possibly a loose manifold bolt where it bolts to the head?
Try spraying some carb cleaner around the intake manifold and see if the idle changes when you do it.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...