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

[Douglas Shook]

Hi all,

I second Brian's prognosis. Whenever something stops working suddenly, the
first place to look would be what has happened to it most recently. The
fence in the blade may have sheared the pin, but it should not have shorted
the coil or fouled the plug. The fact that the plug is wet, there is spark,
but it will not start all would indicate the timing is off from a
sheared pin, assuming that it has a pin. Some engines may not have one.

On Wed, Jul 2, 2025 at 12:54 PM Brian Borgstede <bborgstede at charter.net>
wrote:

> If it were an ordinary push mower, I would say flywheel key. Fairly common
> if you hit something and stop the engine suddenly. It might have a spark,
> but the timing would be off. Just a thought.
>
> Sent from my iPhone
>
> > On Jul 2, 2025, at 1:55 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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