Youâ??re almost certainly looking at a magneto ignition system in all of those
small enginesâ?¦ Bog simple indeed!
â??
Tom Coradeschi
tjcora@icloud.com
> On 18 May 2021, at 10:53 PM, Bob Spidell <bspidell@comcast.net> wrote:
>
> re: "... they are using iron mass of the blade as part of the inductance
> field to signal the position for coil firing ..."
>
> That was my guess, based on what they told me; the motor was otherwise in
> good shape (how I managed to get it back together still amazes me). IIRC, it
> was a Tecumseh 5HP engine. The name of the shop was 'Knife Stalkers.'
>
> I'm looking into possibly rebuilding a couple chainsaw motors, the coil and
> 'points,' or whatever solid state switch they are using, are very simplistic.
> There may be a magnet in the crank gear to trigger the spark. I use Pertronix
> in several cars and, except for one they have a cap on the distributor cam
> with magnets to fire the electronic switch. Surprising to me, the one on my
> 4-cyl Austin-Healey doesn't use the cap, apparently the lobes on the
> distributor cam are sufficient to fire the Hall Effect sensor.
>
> bs
>
> On 5/18/2021 7:38 PM, old dirtbeard wrote:
>> 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> 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@autox.team.net> on behalf of Karl Vacek
>>> <stearman809@gmail.com>
>>> Sent: Tuesday, May 18, 2021 3:52 PM
>>> To: 'shop-talk@autox.team.net' <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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>
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>
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