Misfire/small backfire on my 89 gt.

Hey guys,
Well title says it all, stang is misfiring on left side of motor.(left side if your driving) New valve in number 5 piston(bent). it ran fine after that, just throwing it in there. I have an older spark plugs in that side but it does not looked fouled at all. I can hear the difference is piston firing listening to the tailpipes. THe left side is loping way harder, and more rough. When I rev it, I get a small backfire out of the left side. It running like crap. Do you think it could be that spark plug? It's a motorcraft, The other 7 are brand new platinum double tips. SHe did not misfire after I changed them. I droped one in the engine, never to be found, that's why ther are only seven new

Sorry it's so chopey, I'm tired as all H-E double hockey sticks. Thanks.

Yeah, that plug is in there somewhere. The plug wires are pretty new (4 months), they are frpp 9mm. Iwas thinking it could be an injector, but they only have 28k on them(frpp 24#'s) Could be though. I wish I had a code logger, I could just run the codes, and she what she says. I'll try the wire test, don't know why Ididn't try that yet. Thanks alot for your input.
Bryan