Magnetos still power the vast majority of small aircraft piston engines;
they're a bit more complicated though. Mags in good nick will throw a
really hot spark; they're always 'hot' and the only way to disable them
is to ground the primary coil by a 'P lead.'
On 5/18/2021 8:03 PM, Thomas Coradeschi wrote:
> 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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>> _______________________________________________
>>
>> Shop-talk@autox.team.net
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> _______________________________________________
>
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