On the old carb. motors, it could be the trigger or one of the power packs.
If it is the trigger, it will be the opposite sides that are not firing. Example, #1 and #4 or #2 and #5 or #3 and #6.
If it is the power pack, it can be any of them on the same side.
Those are issues that I had with my old Mercury.
Lots of good info. on outboards on the link below.
http://www.themarinedoctor.com/cgi-bin/YaBB.pl?board=MM