Re: can you explain the mechinical aspect of the interrupter switch function
...and when the switch is wide open...
This has me concerned. You mention that when the switch is "wide open" the engine struggles. This is not right. The end of the ignition switch interrupter arm where the roller tip is, should be centered in the V-groove of the A-arm at all times when running the boat on muffs. If its not, then you probably have a bad lower shift cable (likely), or the shift cables are out of adjustment (unlikely unless you've been diddling around here). If the shift interrupt switch roller tip is centered on the A-arm, and the engine wants to die, then the switch is probably bad. I've seen the old style switch (with the arm and roller tip, not the plunger type) develop a resistance to ground internally when in the open position. This shift interrupt switch applies or 'switches' a ground onto the ignition coil which kills the motor. If the switch is in the un-switched or open position (centered in the A-arm V-notch) and you disconnect either wire to the switch and notice an improvement on how the engine runs, change the switch.
What is somewhat more likely is that your lower shift cable is worn out. When the cable wears, it becomes stiff, and the core won't slide smoothly inside the sheath. When this happens, it will cause the shift interrupt switch to apply a ground to the ignition coil when it shouldn't. To test this you have to have a load on the prop (boat in the water). If you are in the water and shift the boat
from forward
to neutral while watching the A-arm, the A-arm should move as the gear shift is placed into neutral. The movement is caused because the shift cable core reaches its limit of travel and begins to pull on the cable sheath. When the sheath is pulled it causes the A-arm to go off center, which causes the shift interrupt switch to apply ground to the ignition coil. The instant the engine starts to die, the gears in the lower unit come apart (because the A-arm spring and cable sheath are pulling on them), the cable sheath no longer has tension on it and the A-arm centers and the shift interrupt switch removes the ground on the coil. Happens quick, but you can see it if you have someone move the gearshift while you're back there watching. What is probably happening is that, because the lower shift cable, the one attached to the A-arm (which is spring loaded in both directions) is worn out and sticky, won't let the A-arm move quickly and freely back to the center position. This then has the effect of either stalling the engine, or making it hard to get back into neutral. It doesn't matter whether you are coming from forward to neutral or reverse to neutral, the prinicple is the same. And the switch doesn't do anything when going
into forward or
into reverse. Only coming into neutral does the switch work (because the gear teeth in the lower unit are designed to grab one another when going into gear - also referred to as a clutch, and stay engaged).
You won't see the A-arm move with the boat on the trailer because the lower shift cable sheath has to have a load on the prop in order to move the A-arm. Air isn't enough load.
One more thing - I think that when the lower shift cable goes bad it causes the A-arm to move sluggishly, thus the shift interrupt switch to not have a good snapy on/off effect. This 'lazy' operation of the lower shift cable then causes the shift interrupt switch to see a longer duration of electrical current through it (from the ignition coil) which then causes the switch to fail. Its designed as a momentary switch, not as a constant-on current-carrying switch. You could be facing two problems making it more difficult to troubleshoot. Both the cable and the switch might be failing.
