Have a 1997 F-150 4.6l 4x4, runs rough misfire # 8 has 120 lbs compression, plug is black when i take it out, swaped plug with another hole same problem, took plug out and grounded it out and it has spark but could it be to weak?

 

One of the reasons for the plug being black is excess fuel getting into that cylinder. That can be caused by a leaking injector for example. Also if the spark available to that cylinder is strong enough to look good with no compression it just might not be strong enough to fire the plug when under compression. Switch the plug wire with another and see if that changes anything.