ground test to plugs

[rollingsculpture]

well heres one , i have been running my old truck for a yr and a half and have always felt it was a little under powered have set timing and am using a decent zenith carb everything is set up as well as they are going to be like point gap and spark plug gap i have also changed to a b cam for the distributor and find the truck seems to run smoother with . heres the problem when i do a ground out test to the plug wire copper strip using a screwdriver grounding to a headnut i find cyl 1,2,and 3 to drop in idle and the engine runs noticeably off but on number 4 nothing changes when grounding . spark seems weak red yellow color instead of blue white but when not grounding the engine does not seem to be missing , so what am I MISSING.


thanks