[Shop-talk] Mower blade orientaaion relative to piston

[Pat Horne]

I’ve had a no start condition a couple times with what appeared to be a good engine. Turned out the flywheel key was bad, it had sheared. 

Pat Horne 
We support Habitat for Humanity


On May 19, 2021, at 7:33 AM, Bob Spidell <bspidell at comcast.net> wrote:

 All I know is what the 'Knife Stalkers'--they sounded like Russians--told me. I don't have a scientific explanation, only that I tore the engine down and found nothing wrong. I do know that small, basically disposable appliances are built as simply and cheaply as possible (you can rebuild a Stihl MS170 for about $50). The blade wasn't broken--I would have spotted that--but a crack could disrupt a magnetic field somehow. Like I mentioned, the Pertronix in my Healey 100 apparently fires off the cam lobes, with no magnet required to trigger (what I suspect) is a Hall Effect pickup.


On 5/19/2021 5:07 AM, DAVID MASSEY wrote:
> I would find it mildly surprising if they were triggering off of the blade but any motor I've worked on is at least 20 years old.  On those motors (a couple Briggs and Scrapiron and a Tecumseh) had the magneto in the flywheel on the opposite end.  They will run without the blade attached.  Triggering off the blade means that motor is a special-purpose model for that application.  The economies of scale favor a more generic design.
> 
> Dave 
> 
> 
> 
> -----Original Message-----
> From: old dirtbeard <dirtbeard at gmail.com>
> To: Bob Spidell <bspidell at comcast.net>
> Cc: shop-talk at autox.team.net
> Sent: Tue, May 18, 2021 9:38 pm
> Subject: Re: [Shop-talk] Mower blade orientaaion relative to piston
> 
> Hi Bob,
> 
> I believe you, of course, but I have never heard this before.  I have a Craftsman 21" walk behind and I usually sharpen the blade and do maintenance this month, so I think I will see if the engine will fire with the blade removed for sharpening.
> 
> For this to make sense to me, there would need to be some sensor on the blade, or they are using iron mass of the blade as part of the inductance field to signal the position for coil firing. The aluminum sump of the motor protrudes through the steel deck of the mower, so I suppose it is possible. There is quite an air gap between the blade and the sump of the engine, however.
> 
> You have me very curious and I will let you know if the mower will fire without the blade attached (if it does, I will not let it run as it is hard to predict what would happen without that flywheel attached to the crankshaft).
> 
> best,
> 
> doug 
> 
> On Tue, May 18, 2021 at 5:35 PM Bob Spidell <bspidell at comcast.net> wrote:
> Possibly of some interest: I had a Craftsman walk-behind that quit running suddenly. I tore the engine down, didn't find anything, then finally took it to a repair shop. They told me the blade had a crack in it that caused the problem; supposedly the blade is part of a timing loop that fires the spark. Sort of makes sense if you think of the blade as a flywheel with a position sensor on it.
> 
> Bob
> 
> On 5/18/2021 2:55 PM, Brian and Wendy Warrick wrote:
>> I run Snapper walk behind mowers and have never gave it a thought. I can't see why it would matter. If it did, they would have designed the adapter differently.
>> 
>> Brian
>> Nampa, ID
>> 
>> From: Shop-talk <shop-talk-bounces at autox.team.net> on behalf of Karl Vacek <stearman809 at gmail.com>
>> Sent: Tuesday, May 18, 2021 3:52 PM
>> To: 'shop-talk at autox.team.net' <Shop-talk at autox.team.net>
>> Subject: [Shop-talk] Mower blade orientaaion relative to piston
>>  
>> Last time I sharpened my 21” walk-behind mower blade I thought of something that’s never occurred to me, after using the same brand of mower for 25 years..
>>  
>> On a Snapper, the blade can actually go on any way onto the square adapter keyed to the crankshaft.  The mounting point is square, no pins, no longer side, nothing.
>>  
>> One way would be aligned with the cylinder at TDC/BDC, and the other way would be across.  It’s been off many times and there’s no way to tell how it was mounted originally.  The manual is silent on this.
>>  
>> Power stroke shouldn’t matter, but maybe some dynamic issue one way or the other?
>>  
>> Thanks!
>> Karl
>> 
> 
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