I agree with
withamc on this one. The fuel pressure and/or regulator should be first looked at if you're having this kind of problem. 35 would be a good place to start with pressure....you did get an adjustable regulator, didn't you? And the gauge to check it?
I think you may have accidentally been stuck with a MAF for 19 pounders, too. To be sure it's a fuel problem (caused by an incorrect MAF and wrong fuel pressure), read your plugs. You may want to cruise on over to the idle reset procedure on corral.net (?). Did you reset the computer by disconnecting the battery for 5+ minutes? Good luck.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch