This isn't a response to anyone, I just volunteer it for searchers
like me. I recently bought an old 40 horse Johnson outboard. The
wiring was all removed and that usual separately-mounted junction box
was gone. I went into it cold, thinking like it was a car engine that
I was about to wire. Wrong. I saw sparks and smoke so I backed off to
google it all for a while.

I soon became aware that the motor needs nothing from the outside to
run. The magneto does it all. It then occurred to me that the
electric starter was the only reason for having a 12 volt battery. So
far so good, I even found a Johnson shop manual that covered it. The
manual said that the marine solenoid was "different" from an
automotive solenoid, and warned that under no circumstances should I
use the automotive type solenoid. It didn't give details, and so I
didn't know if the solenoid that arrived un-installed with the engine
was marine or auto.

I went to google groups and searched extensively for information on
the difference. Although I didn't know it until I solved my mystery,
most of the guys who chimed in on the question were just giving
guesses and opinions. Like, "They just print Marine on them so they
can sell them for more." Or, "There is no difference." Eventually I
got the answer using just a small clue here and there, a few words
from this or that adviser. It was like working a jigsaw puzzle.

As you all know it is a very simple circuit. The windings in the
solenoid need battery at one end and ground at the other.

On a marine solenoid, battery is wired to one of the small posts, and
ground comes in via the starter switch, etc. to the other small post.
That's it, the two small posts are both ends of the winding.

In an automotive solenoid battery comes in to one small post via the
starter switch. That is one end of the winding but the other end is
connected to the solenoid case, and is thus grounded permanently to
the auto chassis. The other small post, if there is one, will become
energized along with the starter cable during the time the solenoid is
operated. I was told that this supplies a full 12 volt jolt for spark
during cranking. It bypasses the ignition resistor. (I suppose that is
where I got my smoke and sparks from. Luckily, when I jumped back I
jerked my jury- rigged wires loose.)

I did use that automotive solenoid. I wired ground to the solenoid
foot-mount from the starter switch. Of course I will insulate the
mount when I make it permanent.