What was wrong with this mower?

jdtm said:
I think "standoff voltage" would be more accurate than stress. From an electrical perspective the idea is that the insulation will be more likely to fail due to the need to withstand greater voltage. From your later comments, you seem to agree.

Widening the gap increases the breakdown voltage. Once the spark is started, the voltage drops to nearly 0 even if the gap is made much wider.

Current has to go somewhere since it is a flow of charged particles, but that is different from voltage. Voltage does not have to go anywhere. It seems perfectly content not jumping out at an unsuspecting person looking at a wall outlet. However, forcing equipment to operate at higher voltages than it was designed to tolerate, such as using a coil/magneto with a wider spark gap will increase the demands on its insulation, which can cause the failure you described.
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I stand corrected
Thanks for doing that
Some times the brain is a bit too far behind the fingers
I have rebuilt dozens of Lucas mags where the owners fell foul of the "spark intensifiers "
On the propriority testers of this type they are usually just marked "Bad & Good " and the instructions simply tell you to widen the gap during use from the red ( bad ) small gap area into the green ( good ) large gap area while checking the engine rpm
If you get into the green & the revs don't fall off it is good
None that I have seen instruct to widen till the spark fails to occur .
 
jdtm said:
I had an issue with a fix that seemed uninituitive enough to me that it might have some value here. Anyone interested in venturing a guess on what was wrong? Let me emphasize that I have fixed the issue, so the thread is more along the lines of entertainment/education vs. helping someone desperate to get something working.

I was cutting very high grass with essentially your basic 21-inch push mower with no self propel. It was powered by a 190 cc B&S L-head engine. While going through the grass, the mower hit a hidden piece of wood and stalled. After that it would not start. There was a bit of an attempt by the mower to run when the recoil was pulled, but it would not stay running no matter how hard I pulled. I took the spark plug out and saw a typical small-engine spark when I held the body of the plug against the head and pulled the recoil. I also did a compression test and measured a respectable 86 PSI reading. I will also mention that the flywheel key was undamaged. What was the problem?
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So where is the solution posted?
 
I guess we will never know. I think it is better to publish the solution so that we know after the first reading. Time is precious.
 
Bert beat me to this and I agree with him on the sheared or partial distorted flywhell key. I had this happen to me before and I believe the key is made of a softer metal than normal keyway stock so that it does shear when an object is struck.
 
jdtm said:
I had an issue with a fix that seemed uninituitive enough to me that it might have some value here. Anyone interested in venturing a guess on what was wrong? Let me emphasize that I have fixed the issue, so the thread is more along the lines of entertainment/education vs. helping someone desperate to get something working.

I was cutting very high grass with essentially your basic 21-inch push mower with no self propel. It was powered by a 190 cc B&S L-head engine. While going through the grass, the mower hit a hidden piece of wood and stalled. After that it would not start. There was a bit of an attempt by the mower to run when the recoil was pulled, but it would not stay running no matter how hard I pulled. I took the spark plug out and saw a typical small-engine spark when I held the body of the plug against the head and pulled the recoil. I also did a compression test and measured a respectable 86 PSI reading. I will also mention that the flywheel key was undamaged. What was the problem?
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Flywheel key partially sheared.
 
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