On Sun, 26 Apr 2020 11:23:47 -0700 (PDT), wrote:

On Wednesday, April 22, 2020 at 11:12:39 AM UTC-4, Jim Wilkins wrote:
"David Billington" wrote in message ...


When the points open the light goes out at least if you have the test
lamp wired between the dizzy and the coil low tension side to show when
the points are conducting. Your post about the Aussie guy is correct
regarding the way the points operate, I've used static timing many times
for initial set-up on cars.

===================================

If the test lamp is in series between the coil and breaker points it will go
out when the points open and interrupt the coil current, which is when the
spark jumps.

However I connected the lamp across (in parallel with) the points so the
light turns on when they open, and I didn't need to disconnect the coil wire
or disturb the points or distributor position by reconnecting it after
properly setting them.

The coil is a large inductor which converts the energy of current flowing
through it into a magnetic field. When the strength of this field increases
or decreases it acts like a generator that opposes the change in current,
causing the current to appear to have inertia. Current increases fairly
slowly when 12V is applied across the coil by closing the points, and it
tries very hard to continue flowing when the points open to break it.

in the old low-tension (low-voltage) ignition system the points were inside
the combustion chamber. When the points opened the current would continue
between them as a spark, like arc welding, which fired the cylinder.
https://www.gasenginemagazine.com/ga...n-zmgz14jjzbea
I've burned through too much of my daily internet ration looking for an
image of the early Daimler igniter which IIRC really was pointed, thus the
term.

Separating the switching and spark gap functions made both more reliable.
The high tension system you have uses a coil with two windings, a
transformer.

The low voltage or primary winding is matched to the battery voltage and
current, and the high voltage or secondary winding of many more turns of
finer wire is optimized to create a high voltage, low current spark across
the spark plug. Otherwise it operates like the simpler low tension system
whose concept may be easier to understand.

When the points close the current and magnetic field build up during the
"dwell" time, which is part of the distributor shaft cam profile. The grease
in the ignition kit goes on this cam. When the points open the field tries
to keep the current flowing through both windings, and succeeds through the
spark plug gap, where the arc current rapidly depletes the magnetic field's
stored energy. The condenser (mostly) absorbs the current that would have
sparked across and eroded the points.

Then the condenser returns its stored electric charge to the coil, and along
with other inherently capacitive elements of the system causes the coil
voltage and current to oscillate and create radio and TV interference. The
cure is energy-absorbing resistance built into spark plugs with an R in
their designation.

If the gas vapor all ignited instantly it could be fired at piston Top Dead
Center, but since it doesn't (and shouldn't) the spark occurs before TDC to
give the flame time to spread. The timing mark is when/where the plug should
fire at idle speed. As the engine speeds up the centrifugal advance
mechanism rotates the breaker point mounting plate to make the spark occur
about the same length of time before TDC, so a strobe timing light shows the
marks appearing to move on the flywheel.

If the distributor has a vacuum advance its purpose is to advance the spark
further when the engine is at speed but lightly loaded, just cruising down
the highway. This improves fuel economy. Modern engines achieve the same
effect by advancing ignition timing until they sense the loud onset of
preignition ( knock), then backing it off a bit.
https://shop.advanceautoparts.com/r/...a-knock-sensor

When I entered the automotive electronics business in 1973 the above had
until recently been all the electricity an automotive engineer had to know,
since they bought radios from elsewhere. The new engineers Detroit quickly
hired weren't familiar with the many non-theoretical aspects of electrical
engineering such as component tolerance and corrosion of exposed contacts
and the rushed designs of 1974 plainly showed it, though they learned fast.
Some of what they introduced was taken from chemical lab instruments or
military aircraft, but those were much too complex and expensive to use
as-is. I had to learn fast myself in order to build the factory test
stations for the new electronics they were introducing like ABS.
http://www.safebraking.com/before-ab...brake-systems/

Fortunately I had studied material science, combustion chemistry,
thermocouples, hot wire mass air flow sensors etc in college.

Wow! OK more than I can digest. But thanks.
I did finally understand how my distributor works.
So thanks for that too. In the end I just
put it back together. Set the rotor at ~ the right place.
(I took a picture), checked for spark. A yup. And advanced the
dist. til it started. The timing light worked and the
'old girl' started at 30 deg. I set it for the recommended
7-8 deg. And then cranked up the engine rpm. I didn't see any
advancement of the timing. She seemed to run OK between
30 and 7 deg and I set it for 15. (?)

So let me ask a related question. The plugs are always
getting fouled (black fried gunk) on this tractor and
I clean 'em with kerosene. Could I maybe fix that some
with better timing? Though it does other things this
tractor spends most of it's life dragging a mower deck around
the 'estate' (The mower deck is called an estate groomer in the
manual. :^) Sorta medium rpm 2k, 1st or 2nd gear (depending on grass)
the grass is a heavy load on the pto/engine.

George H.
(did you guys do anything actively to reduce the political
crap on this site/net? or was it natural?)

Spark plugs are made in what is called "heat ranges". Read
https://www.gsparkplug.com/shop/spark-plug-heat-range/

--
cheers,

John B.