Rewiring mercury 650 motor

[ShoestringMariner]

At the end of the season I would like to start rewiring a 1975 mercury 650. I had a mechanic told me not to touch it because the wiring needs are very sensitive in regards to both gauge and length.

I don’t really understand this, as long as a gauge and length are replaced like for like, what’s the problem?

I also understand that there’s a few things with this motor such as the wiring to the trigger that can be problematic plus the switchbox(?) itself.

and can I ask, what exactly does the switchbox do?

 

[Chris1956]

Your mechanic is confused. There is nothing special about the Mercury wiring harness. They do need to be in good shape, so they don't short out and destroy expensive ign components.

A Mercury "Switchbox" is a component of the ign. It is fed by either the battery (early models) and the stator (later and new models).

You likely have a battery powered Capacitive Discharge Ignition (CDI) switchbox. It will be fed 12VDC power by the battery, and uses it to charge some capaciters to a high voltage. When the trigger signals, these capaciters will be Discharged into the coli to fire the spark plugs.