I reversed the 3 an 4 spark wires (at both ends) to see if it was a wire, but the symptom stayed with the #4 cylinder-so I guess the wires are oK. Timing light still flashes 6-7 times then off then back on 6-7 times . repeats but not exactly the same every time. The other cylinders I checked are all right. Does it matter that the plug is carbon covered as to whether the timing light would flash or not?