[Shop-talk] Mower blade orientaaion relative to piston

Wed May 19 04:14:34 MDT 2021

Not to mention that there are two magnetos.  You don't want your lawn mower
quitting in mid-air.

On Tue, May 18, 2021, 23:15 Bob Spidell <bspidell at comcast.net> wrote:

> 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:
> > 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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