But here goes.Are you talking dark brown or black crud gumming up you'r spark plug?If so I'm guessing you'r car burns some oil also.If these are both the case,you've got oil fouling of you'r spark plug.This could be caused by you'r piston rings sticking and not sealing properly.Or it could be valve stem wear,and or a bad valve stem seal.You need to run a compression test on that cylinder to determine exactly what's wrong there.