[Shop-talk] Mower blade orientaaion relative to piston

[Thomas Coradeschi]

You’re almost certainly looking at a magneto ignition system in all of those small engines… Bog simple indeed!

—
Tom Coradeschi
tjcora at icloud.com


> On 18 May 2021, at 10:53 PM, Bob Spidell <bspidell at 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 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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