[Shop-talk] It's A Simple Machine, Right?

[Pat Horne]

I’ve had similar in the past, check the flywheel key, mine had sheared & the ignition timing was off just enough. Cheap fix!

Peace, Pat

Pat Horne 
We support Habitat for Humanity


On Jul 2, 2025, at 2:07 PM, Jeff Scarbrough <fishplate at gmail.com> wrote:


I've got a puzzle...

 tl;dr - should I try the plug first, skip ahead and replace the coil, or is there something else I should try?

The patient is a Snapper rear-engine riding mower, a machine that's as simple as can be.  Powered by a B&S Intek 12.5 HP engine, another simple machine.

My friend was cutting his grass and managed to get the grass chute tangled up in some old fencing.  He got it loose, and continued to mow.  After a short distance, the mower quit running and would not restart.

I brought it into my shop and we started troubleshooting. I first tried ether - it seemed like it tried for a second, and then nothing.  Pulled the plug and it looked wet.  Turned the engine over with the plug connected and grounded, and observed a spark.  Not a magnificent spark, but a spark.  

Verified compression, but due to the position of the muffler, I could not get *any* of my compression testers to seal in the hole well.  But it pushed against my finger with some force.

Moved to the carb.  Pulled and inspected.  Float working, jet clear.

Put it all back together, and nothing.  

Pulled the ground wire off the magneto coil and no change.

So, I'm thinking weak coil.  Not completely dead, but not working under cylinder pressure.  Something else I read suggested a bad plug - this one was installed a month ago and has been working.  Date code on the engine indicates it was built on April Fools Day, 2008.

In the era of $15 lawn mower coils, I would throw one on with no qualms.  But at $40 or $50, I hesitate.

So,  tl;dr - should I try the plug first, skip ahead and replace the coil, or is there something else I should try?

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