1973 Mercury 850 Thunderbolt issues

[Hoosierfish]

Just bought an older bass boat with 1973 Mercury 850. Starts good but after a run (15-20 min) and waiting a bit while fishing or whatever it won’t restart. After finally taking it to a mechanic he had diagnosed it as a bad switch box. He said his supplier doesn’t carry these and told me I would have to find a replacement and I could probably change it myself. I have found several used ones with a guarantee so I guess my question is should I go that route? Do these symptoms sound like a correct diagnosis? And what about replacing it with a cdi ignition? Thanks for any help or suggestions.

 

[merc850]

I had the same problem but it was the trigger that was going, before you throw money away use these steps to trouble shoot. CDI sells the trigger and switchbox.

 

[Hoosierfish]

Thanks for the info. So will this test diagnose a faulty trigger even if intermittent? And would you happen to know the part number for the trigger? 1973 Mercury 850 serial # 3498706

 

[merc850]

Yes; Call a local dealer for the part nr. and availability or buy a CDI unit.

 

[opalized1]

I bought aftermarket cdi and it worked great, with the new unit I had replaced the internal wiring loom, so I put the old unit back on and it worked great, in my case it was the wiring loom that had issues, so check it very closely, before spending m oney for something that may not solve the problem

 

[merc850]

And make sure that the distributor is grounded to the block via jumper wire and the block is well grounded.

 

[Hoosierfish]

All grounds seem good. I’ve been inspecting wiring so far it all seems to be ok.