Points and Timing ???

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BearKiller
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Points and Timing ???

Post by BearKiller »

At last night's tractor club meeting, we were discussing the Kohler engine's points and timing relationship and this question came up:

Which advances the timing, less points gap or more points gap ?

Thanks for reading and all help is appreciated.

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cholloway
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Re: Points and Timing ???

Post by cholloway »

Good question... Never thought about it before.
I would think less.
Closing the gap would make the engine fire earlier
before TDC thereby advancing the spark?????
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mrbrown
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Re: Points and Timing ???

Post by mrbrown »

Wider point gap advances the timing. Makes them open sooner. Mike

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Tom Scott
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Re: Points and Timing ???

Post by Tom Scott »

What Mike said. The way to remember this is to understand how a points type ignition works... When the points are closed, the coil is essentially "charging" and internally the inner and outer coil reach an equilibrium. When the points open, this creates an imbalance between the inner and outer coil, so the inner coil's energy is looking for a place to go, and fires. This is my best off the cuff layman's explanation, please forgive any liberties taken with the physics...

The point is, a coil fires when the points are opened, so a wider gap will open the points sooner, thereby advancing the timing.
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cholloway
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Re: Points and Timing ???

Post by cholloway »

Thanks guys... I learned something today.
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BearKiller
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Re: Points and Timing ???

Post by BearKiller »

Tom Scott wrote: The point is, a coil fires when the points are opened, so a wider gap will open the points sooner, thereby advancing the timing.
:beer:
Thanks for that explanation.
I hope I can put the thought in my head into typed words that make sense.
On considering your good explanation, the size of the eccentric and the length of the operating rod are static, thus the travel of the rod is unchanging, regardless of the gap.
Thus, a wider gap is obtained by the points being in closer to the motion of the rod; and, therefore, the points begin to open sooner (and close later) simply from being closer to the rise of the eccentric.
A more shallow gap will open later and close quicker because it does not receive the full travel of the rod.
This is all beginning to make some sense to me.

Do any of you guys use a timing light like that guy on YouTube demonstrates (same engine except on a Wheelhorse) ?
On quick investigation, it looks like an almost impossible task when the engine is mounted on the Cub Cadet.

DaveKamp
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Re: Points and Timing ???

Post by DaveKamp »

Tom's explanation is about perfect... points closed flows current through the coil primary winding...

Here's everything you never wanted to know about why your ignition system never works quite right:

When current flows through a wire, it creates a magnetic field. When that wire is wrapped into a coil, the magnetic field of each turn combines with the other turns, creating a stronger magnetic field. With an iron core in the middle, the magnetic field is concentrated substantially.

When the points open, that magnetic field collapses.

But that's not all- The coil isn't just one coil... it's two. There's the primary, where battery current flows when the contact points are closed... and there's the secondary, which is wound around the core, WITH primary coil... and one end of this wire is grounded, the other goes to the spark plug. Since it's a coil of wire wrapped around a core, any magnetic field that appears on the core, induces a voltage on the secondary winding. An ignition coil is a type of transformer... and they all work in terms of a ratio.

Let's say you have a 12v AC signal... on and off in one direction, then on and off in the other... and your coil has 10 turns of wire around the core.

On the secondary side, you have 100 turns of wire. That means 10:100 ratio. If you apply a 12v AC signal to the primary, the secondary voltage will be 120v.

The trade-off, however, is that if you have 12vAC at 10 amps going in, you'll get that 120v out, but only at 1 amp.

The only difference between a power transformer, and an ignition coil, is a little detail: the input is switched DC, and the output is the plug electrode. It's not designed to transfer power, it's designed to generate a hot spark.

The concept is deceptively simple:

Contact points close, and current flows through the coil. Current doesn't flow immediately, it has to build, because a magnetic field can't be instantly created... so contacts close, and current comes up gradually, until there's full current flow, and a full magnetic field.

Cam lifts points open, so the primary current simply stops... but the magnetic field is still there... just like it's not instantly created, it can't just vanish... it has to collapse... which is what it does, just like knocking the kickstand out from under a motorcycle... and the field collapses about the same way a motorcycle falls over... slowly at first, but then, WHAM.

Now, because there's only about 50 turns on the primary, but there's about 10,000 turns on the SECONDARY, there's a substantial step-up ratio (10000/50=200x).

Normally, one would calculate that with 12v in, the output of a 200x coil ratio would result in 2400v... but this isn't a power transformer, it's an IGNITION transformer.

When the points open,tThe secondary winding is open- it goes to infinite resistance, coil primary current stops... and there's this big magnetic field with no 'kickstand'... and the secondary side, although grounded at one end, is open at the plug gap, on the other.

the magnetic field falls HARD, and in doing so, induces a voltage on that secondary, but there's no place for the current to flow (air gap at the spark plug, right?), so as the field continues to collapse, the secondary voltage keeps rising, and rising, and rising.... instead of stopping at 2400v, it goes to about TEN TIMES' that level, and it's looking for a place to FLOW... because physics requires it... the motorcycle is falling over, it must eventually land.

At the spark plug, there's an air gap. It's actually not air, it's air, or fuel, or carbon, or oil, or some combination of all those. That gap may be half fouled, or it may be perfectly clean, but one thing is for certain- if it's not shorted, it will become the 'war zone' for electricity looking for a place to cross from coil secondary winding, to ground.

Step to one side: Gases. Y'all know about neon tubes... well, most neon tubes aren't filled with Neon... some are, but some are filled with other gases... argon, helium, flourine, nitrogen... when you run electricity through them, they all get excited... they change 'phase', and carry electric current, and when they do, they generate lots of light. Different gasses create different light spectra. When excited, gasses have EXTREMELY LOW resistance. This is what is frequently referred to as 'plasma'.

If Phase change is a confusing term... consider this. Solids melt into liquids. Liquids evaporate into gases. Gases excite into PLASMA.

Boiling water to make steam... is phase change. Condensing humid air to make water, is phase change.


Gases all behave in a similar way... they don't WANT to carry much electric current, but get the voltage high enough, and they 'break over'... they change from a resistive state, to a conductive state... and the voltage at which they break over, is determined by the type of gas, and the distance between the electrodes... and the SURFACE AREA of the electrodes. It's all about the concentration of voltage, over a small space.

Try to use a pair of pin-heads facing eachother to create an excited gap, then do the same by flipping the pins around so pointed ends face, and you'll find that the arc appears at a much lower voltage!

Now, when you put the gas under pressure, it becomes more difficult to excite into a current-carrying state... but like any other circumstance, once the gas ese excited

As the coil field collapses, voltage at that electrode starts to rise... and as the field falls, the voltage continues to climb, higher and higher. Eventually, voltage is high enough that the gases excite, and now you have not only the light of the spark, but a short time of LOTS of current... and when it does, there's a really substantial rise in temperature where that spark occurs.

Now for the details that you really need to know, in order to understand why things don't work right:

When that spark plug, plug insulator, cap, or wire is dirty, it offers a pathway for rising coil voltage to 'leak' back to ground. When a leak occurs, the collapsing energy of that coil HAS someplace to go, so the voltage never rises... it just leaks away.

When the spark plug has liquid fuel on it, or carbon soot... or droplets of water from that hygroscopic ethanol-blend you shouldn't have put in there last fall... the coil energy is shorted to ground before it can ever build to a firing level. Clean the plug, and use clean fuel!

The coil's primary is getting current switched on and off. When the points open, and the field collapses, it's not JUST the secondary that gets whip-cracked with high voltage... the PRIMARY (where your 12v WAS) does too! Those points become the blood-brother to your spark-plug gap... and if the collapsing field is TRYING to jump across the plug at 20kv, but instead, the POINT GAP arcs over at 100v... then your precious magnetic field discharges through the arcing POINTS, not the PLUG. Not only does this eat most of your spark energy, it also burns up the contact points, leaving chunks of non-conductive crud between 'em.

So there's a gadget across the points... called a CONDENSER. The electronic term we use NOW, is a capacitor... and like a battery, it chemically stores a charge... or absorbs a surge, WITHOUT passing it through... as long as it's not shorted, leaky, or insufficient.

If you ever look at your points when the engine's running, and you see sparking, change the capacitor.


Now on to the rod and cam... and arm and spring. The cam pushes on the rod, which lifts the breaker arm opening the points. The spring pushes the points closed, but the rod has to go back too... so the breaker arm is not only re-closing the points, it's PUSHING THE ROD back.

Look at the points, you'll see the electrical connection for the point contact, is at the base end of the spring. Coil current flows from that terminal, THROUGH THE SPRING.. to the contact points, then ground.

Clever idea... good idea... BAD IDEA. (sigh). Why?

Because when you flow current through steel, it gets hot.
--When you heat up a spring, it loses it's temper.
--A temperless spring is FLACCID.

When you leave the key ON, but the engine is stopped, and the points are closed, there's FULL BATTERY CURRENT FLOW through the points and coil...

...which kills the coil, and overheats the spring. (sigh).

Did I mention that the spring is responsible for PUSHING the points closed AND pushing the rod back?

When the spring is weak, it can't do that with much authority. Sure, it'll close the points... but they'll make lame contact, and because the spring is sacked-out, it'll take FOREVER to push that pushrod back against the cam. When you open the throttle, the engine will get up to whatever speed those points can still manage to push the rod shut, and when they hit the speed-limit of that spring, the points will no longer open and close in sync with the cam lobe, it'll spit, sputter, miss, and fart in your face... and all the viagra in the world won't make it youthful again.

(btw... this is why, when doing a rebuild of an old engine, guys always compare spring heights AND measure spring strength. Springs that sit in stuck engines for 50 years tend not to be so youthful, particularly when the valve is in an compressed-spring state for all that time. Fresh springs are the Cialis of an old flathead...

Now... that pushrod is NOT QUITE LONG ENOUGH to always contact both the cam, and the points, at the same time. If it WAS, then the points would never be tightly seated on the contacts, right? That means there's something called 'lost motion' occurring. The point cam bumps the rod, the rod moves, then comes into contact with the point arm, then it starts pushing on the arm, and eventually, lifts the breaker points. All the motion that occurs PRIOR to the points opening, is the 'lost motion'

Lost motion is absolutely necessary. Without it, the point contacts will not seat TIGHT. If they don't seat tight, they will not carry coil current well enough, or long enough, to build an effective magnetic field in the coil.

Remember how I said the coil doesn't collapse immediately? It doesn't build immediately either. The points have to be closed tight for some finite amount of time, BEFORE a strong magnetic field will be ready for the next ignition event. The amount of time that the points have to be closed, in order for the coil to fire, is a function of the coil's design, the contact points' effectiveness, the coil primary wiring quality (from the battery to keyswitch, to coil) and finally... the SPEED at which the engine is running.

This time... the time that the points are closed, is called "DWELL". Dwell is expressed in 'angle'... degrees... that's the degrees of crankshaft rotation that the points are closed PRIOR to them opening. More point gap = shorter dwell. Less point gap = greater dwell.

Having the gap closed too long, is hard on the points, point spring, and coil. Having too little, means you won't get good spark.

Anybody need a bathroom? Let's take a break while I wipe off the whiteboard...
Yes, I'm a Mad Scientist... but I'm usually happy, even when things ain't goin right.

BearKiller
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Re: Points and Timing ???

Post by BearKiller »

DaveKamp wrote:Here's everything you never wanted to know about why your ignition system never works quite right:...
Thank you, DaveKamp; this is much appreciated --- very much appreciated.

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