I don't use one with the MC2100 on my 4.2L. The adapter plate used to mount the MC isolates the carb more than the stock carb ever was and mine seems to run fine.
Maybe post up with what your jeep is doing and we can help isolate the issue. Typically, not having a spacer doesn't make a vehicle "run poorly". I'm guessing the issue is something else, although it could very well be fuel related.
'84 CJ-17: 17" stretch, locked Toy axles, 5.5" lift, FI, AX15, D300 w/twin sticks, boatsides & junk.