vacuum switches on Big Twins?

[Robert Ellis]

Hi,
I've recently come to own a quantity of mid to late 60s evinrude/Johnson motors (33 & 40 hp). I've started working my way through them ... rebuilding the ignition system, replacing the impellers, adjusting the carbs etc etc.
My question has to do with the vacuum switch on the starboard side of the motors and the what I presume is some sort safety switch up above it along the armature plate. On several of these motors, the lead wire (which always seems to be very corroded ... flaking plastic sheathing etc) going to this 'safety' switch has been pulled off of the switch and permanently attached to a ground screw. These motors seem to run just fine with this configuration but I'm wondering what safety feature is being bypassed? I guess what I'm really saying is I don't understand this circuit (or the vacuum switch circuit for that matter). Could anyone help explain these circuits - using the simplest terms possible and advise me re these bypassed switches?

 

[HighTrim]

Simply put, the vaccuum switch will kill a cylinder when there is excessive vaccuum in the intake manifold. This would happen if you blew a prop hub say, and the motor revved up. The increased vaccuum in the intake manifold would pull the plunger in the switch, making a circuit, which grounds one of the points.

The safety switch prevents starting the motor with too much throttle or in gear.

 

[Robert Ellis]

Ok, thanks for that. I think I understand - the switch allows the starter solenoid access to ground when in the proper position and cuts off the ground terminal when open? Do you think the fact that the wire leading from vacuum switch to the safety switch is invariably in very poor shape is simply because it is so close to the power head? Would replacing it with say 12 gauge automotive wire be a sensible choice or would a more heat resistant wire covering be better (not that I know what that would be lol)?