[Shop-talk] Mower blade orientaaion relative to piston

Wed May 19 06:32:12 MDT 2021

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 
> <mailto: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>
>>     <mailto:shop-talk-bounces at autox.team.net> on behalf of Karl Vacek
>>     <stearman809 at gmail.com> <mailto:stearman809 at gmail.com>
>>     *Sent:* Tuesday, May 18, 2021 3:52 PM
>>     *To:* 'shop-talk at autox.team.net
>>     <mailto:shop-talk at autox.team.net>' <Shop-talk at autox.team.net>
>>     <mailto: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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