1982 Mercury V-150 no spark on one cylinder

[r_l_anderson]

I had no spark to cylinder #6. I switched coils between 5 and 6, still no spark to 6. I didn't know if the switch box could cause no spark to only one cylinder, so I switched them last night.

I expected that if the switch box was the problem, that now #5 would not fire. But after switching the switch boxes, now #4 is not firing.

Any ideas? Can a switch box be the cause of a single cylinder not firing? If not, where should I look?

Also, on the four wires grouped together at the top of the switch box, the second wire from the left is black and yellow (maybe black and orange): What is that wire for? If it is engine shut-off related to tilt angle, can I leave it disconnected? Thanks.