[spdbrake]

I think you missed my point the 605 plugs you have installed are too short and have the the wrong mating seat for your heads. Most likely the cause of your exhaust popping due to incomplete fuel burn.

The conical (tapered seat) plugs are usually found on GM engines. The 605 plug you are running do fit 4 cyl Ford Rangers and some V6 Mustangs Taurus cars though.