Hello i have a 92 stang and i have replaced coils,wire, plugs and it ran great but at times it runs great but other times it runs crappy some backfire when i hit the gas pedal. i noticed that the 2 rear plugs always have gunk on them. i clean them again and again but have this continous problem. does anyone have any ideas as to why this is happening? thanx for all replys.

 

What engine do you have? Is it stock?