Reply To: 1970 4 HP, one cyl American Made Mercury

[David Stanton]

I think I found the smoking gun!
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All the stator resistance checks in the manual tested good. I tried another two coils out of old 9.8s that ohm-tested good. Still not spark.
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I put a voltmeter on the positive (green) wire to the coil and disconnected the negative (white) coil wire; then the negative voltmeter lead to ground. When I turned the flywheel, flicking it by hand, The voltage would come up to several hundred volts DC, and slowly bleed down through the internal 5.6 Meg bleed resistor, an probably some voltage bled down through the internal resistance of the Ohmmeter. All this was as expected after analyzing the circuit. I then put the voltmeter negative lead to the disconnected white wire that goes to the points. Got a similar same high voltage response, as expected. The timed bleed down showed the internal capacitor was OK. I saw this voltage build and decay on an oscilloscope also
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When I put the negative wire back on the coil, No voltage read anywhere with the low impedance load of the coil on it.
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I put the ohmmeter across the negative wire after disconnecting it (this goes from coil to points) and other lead to ground. I had checked this days ago when Dave Bernard suggested the white insulator, and it was an open circuit then as it should be. I was on an Rx1 scale on a digital Ohmmeter and it read out of range for the range selected, so that’s high resistance – looked OK…..well….. today I checked it with an analog Ohmmeter that happened to be on a higher scale and I’m reading a constant approximately 20,000 Ohms that has to be somehow bridging the white insulator. When points are connected to the coil, the path from capacitor to + coil through the coil to the – coil terminal and then through the 20 K to ground cannot let the .5 ufd capacitor charge up! My scope showed only a couple tenths of a volt charge. It all makes sense now. So in a couple days when I have time, I will pick that white insulator apart and replace the white wire’s path around it or make a new insulator – anything to get rid of the 20 K Ohm to ground leakage path!
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So I learned to do continuity checks for circuits that should be infinite resistance on a higher meter scale. As frustrating as it was, it was interesting to do the analysis I did working through this, as I now have a good understanding of how each part in the CDI ignition works, including what’s in the stator and how that works. I will not put back in the coil that was originally there that shows an open secondary winding.
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Again -Thanks for all the help!
Dave