Re: wiring diagram 20 yr old 50 merc.
Your motor is in fact a 1973. '73s serial number range is 3478003 - 3816307. I have a '73 Thunderbolt 500 also, but haven't had any issue with spark as of yet. I've done a fair amount of reading lately on here as mine is not currently "up to par" and there are other possibilities aside from wiring. Hopefully someone chimes in and can let you know what order to troubleshoot in. Otherwise, if you search around the forum, there are threads with the same issue. They might be a different motor i.e. Thunderbolt 650, Thunderbolt 800 etc, but the trouble shooting will be pretty much the same.
