Point gap vs timing

In my continuing lesson on how an advanced engineering degree enables the recipient to understand nothing, I think I have finally stumbled upon several truths.

One (and please tell me if this or any of these are correct) is that late timing will vastly reduce my Trac Clipper's power, unto bogging down on my test hill and in addition will cause carbon to form on the spark plug.

Two: The point gap is important not because it determines something strange and electrical about the ignition points, which are just a simple (but very sturdy) switch, but because it is a switch that is operated by a cam. The gap in the points is, in fact, a simple means of properly positioning the 'cam follower,' which is that bit of brown fiber stuff that drags against the cam. Thus positioned, the points will open at approximately the desired position of the flywheel at such time as the lump on the cam contacts the follower. Further adjustments can then be made by adjusting the position of the plate upon which the points assembly is mounted.

Three: my point gap is probably too small, which is why, according to the Sears timing light I've just discovered in the garage, my timing is way, way too late. I'm getting a reading of maybe 2 degrees before top dead center at the maximum advance available from the slots in the magneto mounting plate. Myron's Mopeds specifies 14-16 thousandths of an inch, and I think this sort of engine likes a 13-17 degree advance.

Four: the holes in my flywheel are carefully positioned such that neither the point gap nor the flywheel position when they open can be observed without x-ray vision. I regard this as some sort of divine revenge.

Further enlightenment and/or correction would be greatly appreciated.

Mark Kinsler

Re: Point gap vs timing

Varying point gap can change timing by as much as a couple degrees , advance or retard .

Re: Point gap vs timing

Here's a tip. Always set your points gap to the highest spec of the recommended range. It works out that as the rubbing block wears the gap will decrease and the timing will retard.

Re: Point gap vs timing

Dirty30 Dillon /

Just a note, adjusting the points gap does not change the location of the cam follower on most bikes. The follower is locked into position via the post in the cases.

I would double check your number. Unless you have the incorrect points, no amount of gapping should be able to mess up your timing that much.

Re: Point gap vs timing

Overpriced Parts /

> P D Wrote:

> -------------------------------------------------------

> Varying point gap can change timing by as much as a couple degrees ,

> advance or retard .

On a puch it makes a big difference it can change 20° maybe more, each thousand of a inch is a few degrees

Re: Point gap vs timing

On this bike the cam pushes a little fiber tab mounted on the same arm upon which the moveable contact of the point set is affixed. Thus if the point gap is set too small the tab (which I've called a 'cam follower because that's what it is) will stay far away from the rotating cam, only making contact when the piston has moved too far toward top dead center. The points will open for a short period (called the 'dwell angle') and then close once again as the lump rotates past.

If the point gap is set too wide, the points won't ever close, which stops the spark altogether.

Since my only feeler gauge has blades that don't fit comfortably through the flywheel holes I shall find a piece of steel clock pivot wire about the correct diameter and use that.

I shall adjust and report. The timing light is a remarkable piece of equipment.

Mark Kinsler

Re: Point gap vs timing

Dirty30 Dillon /

Yep, same as most bikes. Adjusting the gap moves the stationary side of the points. The cam follower will always be resting against the cam in the same position, given the physical limits of the adjustment and the wear on the follower.

Altering dwell angle is where timing changes come in.

Re: Point gap vs timing

Richard Eberline /

Points, is the power of the ignition system. The points is primary reason for kilovolts or KV. A stronger spark is the end result.

Re: Point gap vs timing

I thought as much. It depends on the shape of the cam, which I suppose varies with the particular engine.

M Kinsler

Re: Point gap vs timing

> Mark Kinsler Wrote:

> -------------------------------------------------------

> The points will open for a short period (called the 'dwell

> angle') and then close once again as the lump rotates past.

I don't know what ignition system your bike has, but in a battery/coil ignition (Kettering) the amount of time in crank degrees that the contact points are CLOSED is the dwell angle. When the points are closed the coil is being "charged". When the points open the spark is produced.

Re: Point gap vs timing

There are a bunch of different gasoline engine ignition systems, and all of them are pretty strange. Mine is a flywheel magneto, which is extremely clever, cheap, and reliable, but not nearly as simple in theory as some others.

You must begin with understanding how a simple coil of wire and, say, a flashlight cell can produce several thousand volts: enough to give the young experimenter (me) a memorable jolt when the wires are held in the hand and the battery is disconnected from the coil. Every ignition system depends upon this basic principle, for there's really no other way to produce the necessary 12,000 volts for a spark plug.

The ignition points are just a switch whose contacts are exceptionally durable. The coil consists of insulated wire that's wound around an iron bar hundreds of times. We can wind several coils around the same bar. The condenser, or capacitor, consists of two pieces of aluminum foil separated by a piece of plastic wrap, all folded up so it fits in that little round metal case. And there are magnets. That's it.

The magic is in the physics, which tells us that if the amount of magnetism in a coil *changes*, a voltage (electrical pressure) will be produced across the ends of that coil's wire. And: the strength of that voltage depends upon how fast we change the amount of magnetism through our coil.

Mark Kinsler

Re: Point gap vs timing

That is true. When the ignition points are closed in the Kettering system, current from the battery flows through the coil, thus building up a strong magnetic field in the core. This does not happen instantly, for a coil tends to discourage any change in the current that flows into it and will thus produce a voltage to oppose that change. So during the 'dwell' period when the points are closed the coil is allowed to take enough time to build up a substantial magnetic field.

When the points _open_, however, the current is cut off instantly, or nearly so. Again the coil discourages this change in current and tries to maintain it by producing a very high voltage across the ignition points to try to push current from one to the other. In this it partially succeeds, and that's the spark we see between the points when they open. But we really want our spark to show up across our spark plug's electrode, so we place a capacitor between the points to store most of that extra current. We get the current back, however, when the capacitor (okay, condenser) discharges its energy into the coil. That coil is surrounded by a much bigger coil, and the current from the condenser produces a pulse of magnetism in that larger coil to provide our spark plug voltage.

The flywheel magneto is another sort of animal, though the dwell is just as important. That's because our coil is used as a generator coil: it produces current when the flywheel magnet passes by and the points are closed This current circulates through two coils: one is the generator coil, and the other is the 'primary' coil of the spark transformer (also confusingly called the 'coil.') This current makes a strong magnetic field that also soaks through the 'secondary' coil, which has a zillion turns of very fine wire wound on the same iron bar as the primary coil.

When the ignition points open, this strong magnetic field that has soaked through the secondary coil suddenly disappears. Since the rate of change of magnetic field determines the voltage generated at the terminals of the secondary coil, and that voltage is around twelve thousand volts, we'll get a spark across our spark plug's electrodes as well as a dandy shock if we're touching anything we shouldn't.

The trouble is that lots of variations of these systems are possible. Sometimes the primary, secondary, and the generation coil are wound onto one big lump, as in a lawnmower or my old Peugeot moped. Sometimes the generator coil and the primary and secondary coil are separate, as on the noble Trac Clipper.

Right now it's too dark to work on the bike, but I'll open the point gap to some experimental measurement, see how things go, and report.

Everyone has been very helpful, and thank you. Please continue to add advice if you think of anything. What are the symptoms of an engine whose timing is insufficiently advanced?

Mark Kinsler

Re: Point gap vs timing

Still messing with this, Mark? I assume you've made or procured a new woodruff key? The point gap should be around 0.3mm. Basically, the thickness of some cereal box. Place the cereal box in there when the point are at their most open position and tighten down the point screw. Should set you pretty damn close. I have also attached some mechanics notes specific to the M56. They go into great detail on timing these engines.

Re: Point gap vs timing

I am still trying to get my head all around timing and you seem to have a far better understanding of all the science behind it which has been an great read. I have a timing light and a timing tool and I feel like I am going to start using the light more for my bikes as the tool does what you mentioned and make it impossible to see and adjust things when you have it set. The light will require lines on my flywheel which gives me the position I am getting spark at. I can then use a degree wheel to find where the points should be opening and mark that as well. That will allow me to rotate the flywheel to get where I need to make the adjustment and then can fire up and test the change with the timing light.

The feeler type tool that I have is good for finding some measurements but I am finding it harder to set an accurate timing with these days.

Is this sounding correct?

Also I seem to be lost on which direction I would move my stator to advance/retard timing. I feel like it is this. Let's say the flywheel rotates in a clockwise direction. I would rotate the stator counter clockwise to advance the timing and clockwise to restart the timing?

Re: Point gap vs timing

Yes, counter clockwise to advance and clockwise to retard. Your flywheel will have a tiny, embossed arrow pointing in the direction of operation. Batavus has a "tool" for setting point gap and timing. I can't find it ANYWHERE!!! Basically, it is a duplicate of the cam on the flywheel without having a flywheel attached to it. So you can actually see what you're doing when setting all of this. I've thought about finding a spare, identical flywheel and cutting the cam off of it to make my own timing tool.

I've had a huge set back with my Bat. May have soft seized her. My son was taking her for her first voyage with the new carb and he explained the sound as something of a pterodactyl. So, I'll be tearing into her again. I'm pretty damn angry about it as she was FINALLY running beautifully.

Re: Point gap vs timing

> Jay Rivett Wrote:

> -------------------------------------------------------

> Flywheel clockwise : rotate the stator counter clockwise to advance the timing and clockwise to retard the timing?

Correct .

Re: Point gap vs timing

> What are the symptoms of an engine

> whose timing is insufficiently advanced?

>

> Mark Kinsler

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

"An overly-retarded spark timing won't show except as an absence of any evidence pointing to too much advance."

http://www.strappe.com/plugs.html Scroll down.

Re: Point gap vs timing

Pushrod Fifty /

It used to be that a GED was all you needed to set timing and points. But actually I remember doing it in 8th grade.

Re: Point gap vs timing

> Pushrod Fifty Wrote:

> -------------------------------------------------------

> It used to be that a GED was all you needed to set timing and points.

> But actually I remember doing it in 8th grade.

Ha . That puts you at least a couple years ahead of the general population . ;)

Re: Point gap vs timing

> Pushrod Fifty Wrote:

> -------------------------------------------------------

> It used to be that a GED was all you needed to set timing and points.

> But actually I remember doing it in 8th grade.

Well I guess my HS diploma wont work because we were trained in gen ed, and not for working in a trade. I never had a "how to set points" class in any of my science classes. They were more concerned about the sex parts of a plant and the phases of the moon....

Re: Point gap vs timing

Thanks very much for the help and the literature. One of the difficulties in all this is the cluster of theories, many of which are incorrect or, worse, misleading, that have grown up in the 120 years we've been fighting with gasoline engine ignitions. They can be difficult to sort through.

I've found a fine piece of cardboard that measures about .015 inches in thickness. I shall report my progress.

Mark Kinsler

Re: Point gap vs timing

Do not panic. But it's generally best to do test rides yourself. My bike seems to have a weird, intermittent squeal and a still more entertaining clanking, but neither are coming from the engine.

Good luck and keep in touch.

Mark Kinsler

Re: Point gap vs timing

Thank you. That is fascinating and revealing.

Mark Kinsler

Re: Point gap vs timing

Here's a good explanation.

Re: Point gap vs timing

It wouldn't have helped with small engine repair, but I've been trying to reform the general science curriculum into a 'how things work' form to include everything from what holds buildings up, what stuff is made out of (concrete, aluminum, plastic, steel) how an engine works, how we get water and electric power, why sewers allow us to live in cities without dying of cholera, how farms work, and all the rest of the man-made stuff that keeps us alive. Nobody understands what I'm trying to do--they think it's either a shop class or an auto repair class because teachers typically have never wondered how a radio or a telephone works. At least I tried.

Mark Kinsler

Re: Point gap vs timing

> Mark Kinsler Wrote:

> -------------------------------------------------------

> It wouldn't have helped with small engine repair, but I've been trying

> to reform the general science curriculum into a 'how things work' form

> to include everything from what holds buildings up, what stuff is made

> out of (concrete, aluminum, plastic, steel) how an engine works, how we

> get water and electric power, why sewers allow us to live in cities

> without dying of cholera, how farms work, and all the rest of the

> man-made stuff that keeps us alive. Nobody understands what I'm trying

> to do--they think it's either a shop class or an auto repair class

> because teachers typically have never wondered how a radio or a

> telephone works. At least I tried.

>

> Mark Kinsler

Maybe your efforts weren't in vain .

There is a great part of the internet that deals with exactly 'how things work' , from soup to nuts . ;)

Re: Point gap vs timing

I like it mark. I would have been in that class for sure!

Re: Point gap vs timing

Another piece of the puzzle to add into the whole “ spark output “ circuit is the “air gap” of the stator coils to the magneto.

I bought a barn find 1979 Puch maxi with a ZA50 engine.

The magneto was removed from the moped when I bought it.

I was chasing a no spark condition.

Followed the normal troubleshooting steps of cleaning and setting points.

As it turns out, there was no air gap of the coils to the magneto.

From what I found, the coils need to be .020 away from magneto to generate output.

So now that there is the air gap to the coils, I have spark and output for the lights.

I suspect the previous owner at some point, had the magneto off and used a flat blade screwdriver to loosen every flat blade screw behind the magneto.

A Girl Scout cookie box lid, the green o es with the mint cookies, is .020 thick.

Re: Point gap vs timing

↴ ⇊

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Re: Point gap vs timing

> Scott Bard Wrote:

> -------------------------------------------------------

> Another piece of the puzzle to add into the whole “ spark output “

> circuit is the “air gap” of the stator coils to the magneto.

>

That has to be , but , it doesn't have to be .020" .

Actually , the gap can be as close to '0' as possible without flywheel rubbing / touching the coil cores .

Seems that the smallest gap produces the best / most output . (edited)

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