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So I have managed to keep this little Yard Bug running for nearly 20 years because I have another one that I have used as a parts donor. But I've come across a problem I can't figure out. I have no spark. It will crank like champ, but there is no spark at the plug. Removed and tested the magneto coil and read 2.5K ohms. I understand this to be correct. Re-installed at 0.010 spacing from flywheel. I believe this is correct. Is it possible that magnets on the flywheel have weakened to the point that they can't provide enough juice to the coil for a spark?

The very first symptom of the problem: The last time I was using the mower it just quit in the middle of the job. It didn't stutter to a stop like when you run out of gas. It quit like the ignition was turned off. I thought I had a bad safety switch and just bypassed them all. It cranks just fine so I'm pretty sure I did all the bypasses correct.

Any help is greatly appreciated!

If you put the flywheel back on, but leave the small kill wire disconnected and crank, you should have spark, if not then the coil is bad.
What can destroy a coil is somehow voltage gets sent to the coil via the kill wire, so you should make sure that is not happening, as you would quickly destroy a new coil, if you installed one. That circuit is just to ground out the ignition to the block, to stop the engine.
So bare wires, or an incorrect key switch can inadvertently send voltage to the coil and destroy it.

Today’s coils, even those from twenty years ago, cannot be tested with an Ohm meter. With the symptoms you described, I suspect your coil has failed and I would be replacing it.

Fish said:

If you put the flywheel back on, but leave the small kill wire disconnected and crank, you should have spark, if not then the coil is bad.
What can destroy a coil is somehow voltage gets sent to the coil via the kill wire, so you should make sure that is not happening, as you would quickly destroy a new coil, if you installed one. That circuit is just to ground out the ignition to the block, to stop the engine.
So bare wires, or an incorrect key switch can inadvertently send voltage to the coil and destroy it.

Click to expand...

Thank you for this. I never had to remove the flywheel. I hadn't considered leaving that ground wire off of the coil and trying it. I did check out the switch with a meter, and it appeared fine. Should be able to move the coil over from my donor machine easily enough.

Sometimes voltage can be introduced from a spot in the wire harness that is melted, or mouse chewed, or a key switch coming apart internally.

OK, so I'm thinking this must be heat related and it is the coil. I inspected the wiring and none of it is frayed, chewed or otherwise damaged. Then, as Fish suggested, I disconnected the kill wire and checked for spark. I had spark! Re-connected the kill wire and checked for spark. I had spark! WTH? So, going back to my original symptom I think after this thing heats up a bit the coil opens up and stops producing spark thus killing the engine as if someone just turned off the ignition switch. I'm going to take the coil off my donor engine and have it ready. Then I'll try to mow today and see if this thing can make it through the 30-40 minutes it takes.

I'll report back the exact findings when I think I have it solved. Thank you for all the help!

TO diagnose a heat failing coil I install a red nenon tube spark tester to the mower so I can see the tube, then go mow.
When I hear / feel the engine dieing I look at the tester.
If it is not sparking and the engine is rotating above cranking speeds then I can be reasonably sure that the coil is bad .
If it flashes all the way till the engine stops then the problem is elsewhere .

Well I was right. the mower ran for about 10-12 minutes than died. Immediately I pulled the plug and checked for spark and there was none. Replaced the coil with the one from my donor and it fired right up and I was able to complete the job which took at least another 30 minutes. Looks like this little yard bug is good for another season!

Thanks again for the help!