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Welcome to the forum Grumpy! :howdy:

Since it seems to happen when you shut it off, wonder if it could be a short in your ignition switch? Maybe it is intermittently shorting 12V into the kill wire? :confused:
 

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Hmm, just looked at Sears parts and that switch and the ammeter are the only two of the electrical parts showing a current back order. Always makes one wonder if they are having a problem with them when you see that. :D

http://www.searspartsdirect.com/par...P0405132/00001?blt=06&prst=&shdMod=9172736401

Do you have an owners manual for it? If not you can download a .pdf file of it here:

http://www.managemylife.com/mmh/lis_pdf/OWNM/L0406036.pdf

Page 33 has the wiring diagram and shows in detail the ignition switch and also a table of the different switch positions and whether they should read with an ohm meter open or closed.

Your going to need a volt-ohm meter though to test it. The terminal M (one of the two black wires on the switch) should never have 12 volts on it. It will read an open connection to chassis ground unless one of the safety switches are activated or the switch is on the "off" position. In the "off" position or if a safety is activated when in the "on" position terminal M will read a short to ground killing spark. Make mental note of the bold & italics above as that will tell you if one of the safeties is shorted when the ignition switch is in any other position besides "off". ;)

Puzzling thing is if the ignition switch is even momentarily shorting 12 volts into the coil kill terminal one would think it would blow the fuse. Or if it does not blow the fuse at that time, any one of the safety switches when activated would blow the fuse.

But with a new plug there really is nothing else there that could damage the coil that I can see. Strange problem. :confused:
 

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I don't know how that would kill a magneto. But it would definitely change the timing of your spark and also keep the engine from running at all once it got far enough out of time.

Maybe that second replacement coil has not failed after all. Maybe after replacing the sheared key you will be back up and running. :cool
 
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