Are you running 3/4" reach plugs or stock length plugs? Running stock plugs in a TFS head could cause symptoms of running rich.
Is your MAF meter calibrated for 24# injectors? If it was calibrated for 19# the computer is opening the injectors longer than it should. That problem would be corrected by the computer after a while, though. How long have you had the problem?
I have a Pro-M 77 running with 24# injectors and my idle is fine.
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351W 89 Mustang GT Convertible