On Mon, 26 Oct 2015 19:52:29 -0400, Ed Huntress
wrote:

On Mon, 26 Oct 2015 15:58:01 -0700, wrote:



Many of the very simple old 2-strokes just relied on the magnet
*leaving* the field to time the spark. Sort of time, that is. g As
the motor speeds up, the spark timing actually can advance a few
degrees that way.

You may be able to tell from the relative position of the magnet in
the flywheel, to the magneto coil, where the timing is. In any case,
it will give you guidance about setting it on the opposite side of TDC
if you want to run the motor in reverse.
I have worked on a lot of old two stroke motors, motors with cast iron
pistons, completely unbalanced cranks, and grease cups to lubricate
and seal the crankshaft where it exits the crankcase. And I have never
seen an ignition like that. The oldest magnetos I have seen had points
of some type. I can't see how leaving the magnetic field would cause a
spark. Can you explain it?
Eric

Oh, boy, the memory is the second thing to go. g I was confusing
myself with the old low-tension systems with the points in the
cylinder (the "make and break" systems) that we were talking about
here a couple of weeks ago. I got them mixed up in my memory.

As for the relationship of magnet to coil and the timing, though, that
does apply. That will determine the peak output. However, the points
have to open close to that peak, too.

Sheesh. Sorry to be misleading. I hate when that happens.
Yeah, I know all about that failing memory crap. I did know about the
points and magnet timing too. Which is why, for example, when a points
type ignition on a Briggs and Stratton engine won't spark when the
flywheel key shears. Even if it shears only about 1/2 of the key
width they usually won't fire because the spark will be too weak.
Eric