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

[Jeff Scarbrough]

Update to the update:

New coil arrived yesterday.  Installed it today - no spark.  Reinstalled
the old one - weak spark.  Discovered the old one was upside-down -
inverted it and reinstalled it - no spark.  Disconnected the grounding wire
- strong spark.  The old one worked with that wire connected, so there's
another rabbit hole we need to go down for the future.

Meanwhile, now that we had a good spark, we gave it a shot of ether and
cranked.  It ran, but very low power.   So, not the spark.  Checked
hte fuel flow at the carb, still adequate.  So my7 thoughts turned to the
valve train...

Went to take the valve cover off, but it was thoroughly glued on.  So I
pulled the muffler so I could see the valve stem.  It was going up and
down, so I pulled the intake and looked way down in the hole.   The valve
was *not* moving!

At that point, I was confident in prying the valve cover off being a good
idea.  Turns out the top inch of the intake pushrod was lying in the oil
drain back to the crankcase.  The pushrod looks like it was getting crimped
and finally broke where it was getting necked down.  I've never seen
anything quite like this.  I'm not sure now how it was running weakly on
ether or gas if the intake valve didn't open to admit any air or fuel.

More new parts on order.  At least I found something I could verify was
broken.  I call that progress.
[image: RMA mower valve train.jpg]
[image: RMA mower intake pushrod wide.jpg]

On Sun, Jul 6, 2025 at 6:59 AM Jeff Scarbrough <fishplate at gmail.com> wrote:

> Update:  On Thursday, we tried a new plug - no start.  Pulled the flywheel
> bolt and inspected the key - perfect alignment (did not remove the flywheel
> to visually inspect the key itself).
>
> New coil on the way - interestingly, direct from Briggs & Stratton was
> cheaper than buying a B&S-labeled part from Amazon, and presumably has a
> far better chance of being both the correct part and not counterfeit.
>
>
> The saga continues...
>
> On Wed, Jul 2, 2025, 14:31 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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