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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@gmail.com>
> To: Bob Spidell <bspidell@comcast.net>
> Cc: shop-talk@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@comcast.net
>
> 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@autox.team.net>
>> *Sent:* Tuesday, May 18, 2021 3:52 PM
>> *To:* 'shop-talk@autox.team.net
>> *Subject:* [Shop-talk] Mower blade orientaaion relative to piston
>> Last time I sharpened my 21â?? walk-behind mower blade I thought of
>> something thatâ??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. Itâ??s been off many times and thereâ??s
>> no way to tell how it was mounted originally. The manual is
>> silent on this.
>> Power stroke shouldnâ??t matter, but maybe some dynamic issue one
>> way or the other?
>> Thanks!
>> Karl
>>
>
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>
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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.<br>
<br>
<br>
<div class="moz-cite-prefix">On 5/19/2021 5:07 AM, DAVID MASSEY
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:674086163.1251051.1621426055740@mail.yahoo.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">
<div> <font size="3">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.</font><br>
</div>
<div> <br>
</div>
<div style="clear:both"><font size="4">Dave </font><br>
<br>
</div>
<div> <br>
</div>
<div> <br>
</div>
<div
style="font-family:arial,helvetica;font-size:10pt;color:black"><font
size="2">-----Original Message-----<br>
From: old dirtbeard <a class="moz-txt-link-rfc2396E"
href="mailto:dirtbeard@gmail.com";><dirtbeard@gmail.com></a><br>
To: Bob Spidell <a class="moz-txt-link-rfc2396E"
href="mailto:bspidell@comcast.net";><bspidell@comcast.net></a><br>
Cc: <a class="moz-txt-link-abbreviated"
href="mailto:shop-talk@autox.team.net";>shop-talk@autox.team.net</a><br>
Sent: Tue, May 18, 2021 9:38 pm<br>
Subject: Re: [Shop-talk] Mower blade orientaaion relative to
piston<br>
<br>
<div id="yiv5420248293">
<div>
<div dir="ltr">Hi Bob,
<div><br clear="none">
</div>
<div>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.</div>
<div><br clear="none">
</div>
<div>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.</div>
<div><br clear="none">
</div>
<div>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).</div>
<div><br clear="none">
</div>
<div>best,</div>
<div><br clear="none">
</div>
<div>doug </div>
</div>
<br clear="none">
<div class="yiv5420248293gmail_quote">
<div class="yiv5420248293yqt9341191006"
id="yiv5420248293yqt66682">
<div class="yiv5420248293gmail_attr" dir="ltr">On
Tue, May 18, 2021 at 5:35 PM Bob Spidell <<a
rel="nofollow noopener noreferrer" shape="rect"
ymailto="mailto:bspidell@comcast.net";
target="_blank"
href="mailto:bspidell@comcast.net";
moz-do-not-send="true">bspidell@comcast.net</a>>
wrote:<br clear="none">
</div>
<blockquote class="yiv5420248293gmail_quote"
style="margin:0px 0px 0px 0.8ex;border-left:1px
solid rgb(204,204,204);padding-left:1ex;">
<div> 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.<br clear="none">
<br clear="none">
Bob<br clear="none">
<br clear="none">
<div>On 5/18/2021 2:55 PM, Brian and Wendy
Warrick wrote:<br clear="none">
</div>
<blockquote type="cite">
<div style="font-family:Calibri, Helvetica,
sans-serif;font-size:12pt;color:rgb(0,0,0);">
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.</div>
<div style="font-family:Calibri, Helvetica,
sans-serif;font-size:12pt;color:rgb(0,0,0);">
<br clear="none">
</div>
<div style="font-family:Calibri, Helvetica,
sans-serif;font-size:12pt;color:rgb(0,0,0);">
Brian</div>
<div style="font-family:Calibri, Helvetica,
sans-serif;font-size:12pt;color:rgb(0,0,0);">
Nampa, ID</div>
<div>
<div style="font-family:Calibri, Helvetica,
sans-serif;font-size:12pt;color:rgb(0,0,0);"> <br clear="none">
</div>
<hr style="display:inline-block;width:98%;">
<div dir="ltr"
id="yiv5420248293gmail-m_-6852136761353026904divRplyFwdMsg"><font
style="font-size:11pt;" face="Calibri,
sans-serif" color="#000000"><b>From:</b>
Shop-talk <a rel="nofollow noopener
noreferrer" shape="rect"
ymailto="mailto:shop-talk-bounces@autox.team.net";
target="_blank"
href="mailto:shop-talk-bounces@autox.team.net";
moz-do-not-send="true"><shop-talk-bounces@autox.team.net></a>
on behalf of Karl Vacek <a
rel="nofollow noopener noreferrer"
shape="rect"
ymailto="mailto:stearman809@gmail.com";
target="_blank"
href="mailto:stearman809@gmail.com";
moz-do-not-send="true"><stearman809@gmail.com></a><br
clear="none">
<b>Sent:</b> Tuesday, May 18, 2021 3:52
PM<br clear="none">
<b>To:</b> '<a rel="nofollow noopener
noreferrer" shape="rect"
ymailto="mailto:shop-talk@autox.team.net";
target="_blank"
href="mailto:shop-talk@autox.team.net";
moz-do-not-send="true">shop-talk@autox.team.net</a>'
<a rel="nofollow noopener noreferrer"
shape="rect"
ymailto="mailto:Shop-talk@autox.team.net";
target="_blank"
href="mailto:Shop-talk@autox.team.net";
moz-do-not-send="true"><Shop-talk@autox.team.net></a><br
clear="none">
<b>Subject:</b> [Shop-talk] Mower blade
orientaaion relative to piston</font>
<div>Â </div>
</div>
<div lang="EN-US">
<div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Last time I sharpened my
21â?? walk-behind mower blade I thought
of something thatâ??s never occurred to
me, after using the same brand of
mower for 25 years..</div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Â </div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> 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.</div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Â </div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> One way would be aligned
with the cylinder at TDC/BDC, and the
other way would be across. Itâ??s been
off many times and thereâ??s no way to
tell how it was mounted originally.Â
The manual is silent on this.</div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Â </div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Power stroke shouldnâ??t
matter, but maybe some dynamic issue
one way or the other?</div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Â </div>
<div style="margin:0in 0in
0.0001pt;font-size:11pt;font-family:Calibri,
sans-serif;"> Thanks!<br clear="none">
Karl</div>
</div>
</div>
</div>
<br clear="none">
</blockquote>
<br clear="none">
</div>
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