Did a tune up on my 90 GT with a 7,000 GT40 crate engine. 7 plugs looked text book perfect, the one from #4 cylinder had a small amount of black tar like residue. This matches a rich smell I sometimes got from that area of the motor. The injectors are newer than the motor with 5,000 on them. Will a bad injector cause the plug to foul like that? Any insight would be appreciated. Thanks.