Looks like a weak spark caused by failing stator or trigger. See if the spark will jump a 7/16" gap. Do not use regular reach plugs unless manual says so, they will hit the piston top.
A manual would help but in lieu of one I'm posting some starter pages: (before you start soak the cover nut area with a penetrating oil for a few days too loosen it up.)
Almost launch time (except in canada) and many guys will be changing their impellers re-installing their lower units and having shift problems:
This is how I do it in about 60 sec.: Turn the LU shift shaft by hand counter-clockwise until it
stops, then with Vise Grips tight on the bottom of the...
If you put a voltmeter on the + starter terminal and the cowl ground read the voltage while turning over the motor, it should be 10-12 volts if your battery is good. If it does fall in this range your starter probably needs rebuilding/cleaning (brushes). A small engine electrical shop could fix...
Good that you've worked on one before so you know the plug wires screw into the cap, CDI has the engine harness https://www.cdielectronics.com/product/414-2770/, or Marine Engine.com I bought the last one Mercury had this year
I don't know what brand of oil you're using but I've found that Amsoil synthetic smokes less and if you live in the US you can get 100:1 blend which lets you go from 50:1 to 100:1. I was mixing it at 70:1 and of course I used less oil and reduced smoking.
i think you should rewire it - there seems to be some incorrect connections and corroded wires which can cause a major fire. I am wondering about the connections on the terminal block in image 2 they look like they are coming from the stator but one disappears into the cowling and one is...
The Nr. refers to heat range - The middle number indicates the heat range of the spark plug, the higher the number the hotter the heat range the hotter the plug the less it will foul.
According to the Merc manual the 1964-66 3.9's used a J7C plug and the gap is .025"/ the points should be set at .020". Your motor falls into those years.
I'm guessing that the fuel pump/wiring harness/ignition can't be used in your motor, go have a look at it if you can with cowling off.
It might be a 1979 model like this one
