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Print Page oldski-doosleds' Vintage Ski-Doo Message Board
    oldski-doosleds Message Board => Help Desk (Only) => Message started by: GaWajn on February 23, 2014, 08:50:30 AM
	

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Title: Early Rotax Ignition theory
Post by: GaWajn on February 23, 2014, 08:50:30 AM

Hey guys,

I have seen bad or misleading information about this subject so many times ... I thought that we as a group ... could come up with an easy to understand ... comprehensive write up about the subject.

Old (experienced) Rotax Gurus ... please chime in ...

If you find any mistakes ... errors ... or misleading statements ... please help me to correct them. Hopefully ... we will end up with the definetive text on the matter. Also ... if a piece of text is correct but hard to understand ... lets find the words to make it easier for all to grasp.

I will start the ball rolling ...


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on February 23, 2014, 06:57:52 PM

What is needed to make a spark at the tip of a spark plug?

The spark plug tip has two electrodes. Let’s call them the center electrode (incoming electricity flow) and the lower electrode (ground). They do not touch each other. The plug ‘’GAP’’ is the distance between those two electrodes. When a plug or sled manufacturer states that a plug should be ‘’GAPPED’’ at a certain distance, he giving us a specification to set the resistance of the spark plug circuit. The bigger the gap, the higher the resistance value is. The air between the electrodes is in effect, a large value of resistance because air is not a good conductor of electricity.

Think of the center electrode as the positive incoming electricity, and the lower electrode as the drain for that electricity to go back to its source. The air gap is like an open switch in an electrical circuit. For the circuit to conduct electricity, we have to close the switch. If we make the two electrodes touch, that would work to make current flow, but we would have no spark. If we apply a high enough voltage (pressure) to the circuit, the electrons will force their way and burn through the air molecules and find their way to the lower electrode, completing the circuit. Think of voltage as pressure or tension if you will. We need a high pressure (voltage) to force the electrons across the air to the grounding electrode. These electrons burning their way to the lower electrode is what we perceive as a spark between the electrodes.

How do we get a high enough voltage so that the spark is produced between the electrodes?

Since old snowmobiles do not have any source of stored electricity, the machine must somehow produce the high voltage we need to create the spark at the plug electrodes.
When a permanent magnet passes close to a coil of copper wire wound around an iron core, the magnetic field induces a current in the copper wire. I will not go into the theory of why and how this happens. For our purpose, it is sufficient to accept that it does happen this way.

If you take a look at the ignition system in an old Ski Doo Rotax single cylinder engine, it is comprised of a flywheel containing the permanent magnets, both the lighting coil and ignition coil, breaker points and condenser. The flywheel is connected to the crankshaft and spins at the same RMP as the engine. The crank shaft and flywheel are locked together by a woodruff key. This means that the flywheel to crank shaft relation is not a variable regarding the timing of the system.

With each rotation of the crank shaft, the magnets pass four times past the ignition coil.
With each passing of a magnet past the ignition coil, electricity is generated. This electricity is alternating current, not direct current.


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on February 23, 2014, 07:05:06 PM

The ignition coil has two windings. The primary winding is of larger gauge wire and the secondary winding is made with smaller gauge wire. The ignition coil works like a transformer.

Twelve volts is generated in the primary winding and it is then transformed into a much higher voltage in the secondary winding. This voltage (pressure) in the secondary winding is still not high enough to produce current flow and a spark at the plug gap.

What is needed is a way to increase the voltage in the secondary winding to a much higher value. That is where breaker points come in.

The points are wired into the system in such a way that when the breaker contacts are touching together, electricity is flowing through the primary winding to ground, producing a closed circuit. If the point contacts are not touching (open circuit), then no current is flowing through the primary winding.

There is a high voltage (pressure) produced in the secondary winding, but that voltage pressure is not high enough to close the circuit via the spark plug.

I will not explain Ohm’s law in relation to electricity. It is beyond the intended scope of this document. What we need to understand from this law is that there is a direct relation between the values of voltage (pressure), current (flow) and resistance in an electrical circuit.

One interesting effect that happens to the voltage in the primary winding is at the exact time that the contacts no longer touch, the magnetic field in the primary winding of the ignition coil collapses. A magnetic field in a coil can only be generated by the winding if there is current flowing through it. When that magnetic field collapses, a momentary spike (pressure surge) of voltage happens.

Sounds like this is getting complicated right? Not really. Let’s recap.

The magnets have a permanent magnetic field. As they pass the coil, it will induce a magnetic field and current flow in the primary winding, if the contacts of the breaker points are closed (touching). This will in turn produce a magnetic field in the secondary winding of the coil, with voltage pressure. Open the switch (contacts), and the whole thing collapses, producing a voltage spike (pressure surge) in the primary winding.

When the breaker point set contacts open, we know that the voltage pressure in the primary winding spikes to a high value. This higher voltage pressure in the primary winding in turn generates a higher voltage pressure in the secondary winding. This momentary higher pressure in the secondary winding is enough so that the gap at the plug is bridged and a momentary path of electricity flows from the center electrode to the lower electrode. A SPARK is created!
So now we have a means of generating enough voltage to create a spark.


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on February 23, 2014, 07:07:39 PM

Why do we need to synchronise (time) this sparking?

If we do not make the spark happen at the exact perfect time when the engine needs it, problems will occur. If the spark happens too soon in the combustion cycle, the fuel/air mixture ignites prematurely and generates more heat than power. This could lead to detonation and the destruction of components. If the spark happens too late in the combustion cycle, then the energy goes out the exhaust port instead of forcing the piston down to create power.

How do we time this sparking?

You will notice a cam on the crank shaft. The inside diameter of this cam is perfectly round and fits over the crank shaft smoothly. The exterior profile of this cam is not perfectly round. It has a bulge in it. When this bulge is pushing the breaker point arm, the contacts open. As soon as the shaft rotates and the bulge moves away from the breaker point arm, the contacts close. The contacts open and close in relation to where this bulge on the cam is at any given time.

All we need to do is to get the bulge of the cam to open the contacts of the breaker points at the exact time that we want, in order to create the most efficient combustion of the fuel/air mixture.

Great! How do we do this?

All of the components are mounted to a plate. It is called the stator plate. This plate can be rotated a few degrees clockwise or counter clockwise as needed. The effect of this is to have the means of determining when the bulge on the cam, opens the contacts. We just need to follow the timing procedures described in our service manuals to time the engine correctly.

Well we haven’t talked about the condenser yet. The reason we have a condenser in the circuit is to absorb some of the energy at the contact point, eliminating sparking of the breaker points. If we did not have a condenser in the circuit, the breaker points contacts would very soon become pited and no longer function correctly.

Some people think that the condenser discharges to create the spark at the plug. This is incorrect. Condenser discharging is a factor in CDI ignition systems, but that’s a whole ‘nother ball game. CDI ignitions are systems that work somewhat differently than these older rigs.


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on February 23, 2014, 07:14:16 PM

That's about what I have come up with so far.

Others have covered the actual process of timing engines before on this board. It might be a good idea for these procedures to be written up and included in this document??

Then we could host the completed document and refer people to it when they need information about ignition systems in early Rotax's??

That was my original idea anyways.


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Title: Re:Early Rotax Ignition theory
Post by: FrankenCub on February 23, 2014, 09:34:12 PM

Great post to Pin at the top ;)


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Title: Re:Early Rotax Ignition theory
Post by: FrankenCub on February 27, 2014, 02:43:36 PM

[quote author=GaWajn link=board=5;threadid=22202;start=0#msg132173 date=1393515564]
Hhmmm ... not much interest ... maybe it wasn't such a good idea after all ...
[/quote]

I don't know, GaWajn, I think it was a good idea. Might be better off pinned in the Help Desk section though. I believe if someone is having an ignition/no spark issue, if they read through this it will give them a good understanding on how it all works together and will be a help to get them up and running again. Ignition systems may be simple to many, but some only know how to make the smoke escape :)


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on March 7, 2014, 06:26:46 PM

A little more information regarding the role of the condenser in the magneto ignition system ... as found in a Google search ...

''When the points open, the condenser is connected in series with the coil. The voltage/current generated by the collapsing magnetic field charges the condenser. A coil opposes current flow while a capacitor enhances current flow. The enhancement of the condenser balances out or cancels the opposition (choking effect) of the coil.

The result is a faster collapse of the magnetic field and the highest possible voltage generated in the primary winding. It's true that the condenser reduces or prevents arcing at the points but its main function is to provide a circuit path for the coil after the points open and to speed up the collapse of the magnetic field.


... this last part is information I found on the web, from mmany different sources ... and are not my original thoughts on the matter.

This leads me to believe that a weak spark condition could be caused by a faulty condenser, because the magnetic field is not colapsing at it's fastest possible speed ... resulting in a lower voltage in the secondary winding.

A faster field collapse = a higher voltage generated = a stronger spark.


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Title: Re:Early Rotax Ignition theory
Post by: matskidoo1972 on October 30, 2014, 10:23:59 AM

Hey im having a problem with my 1972 299 oly she has great blue spark i put gas i. The carb and she just backfired twice i belive this means my timing is of if i get it right it wont back fire right .. But i also dont wana mess up my point and condencer its all brand new she sparks but just back fires please help me out


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Title: Re:Early Rotax Ignition theory
Post by: GaWajn on October 31, 2014, 04:57:58 AM

Please start a new topic and state your problem in there. People will respond and help you troubleshoot the problem in your new topic. It could very well be a timing problem ... or it could be something else.

This PINNED information topic is not the best place to get answers about a specific problem because most of the crowd that hangs around here don't usually check it ...


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Title: Re:Early Rotax Ignition theory
Post by: craig on December 11, 2014, 07:52:36 PM

question of the day what: on this system is grounded?
I have a 335 motor (pre `71, internal coil). I took apart to reseal. marked stator plate as to not have to retime. put back together, no spark. replaced tired ground wire from breaker, no spark. replaced condenser, no spark, coil, no spark. Put all this new stuff on new plate w/ new points, no spark.
Make flashlite timing lite/continuity testr n find that plate is grounded n so is both sides of breakers. im confused. shouldnt one side of points b isolated n ungrounded so that ground created when points close? if i hook my continuity tester to points, one lead to fixed side n one lead to moving side my circuit should break when points open, my lite stays lit n doesnt change intensity the whole time. (by the way, i love ur repair videos) thanks, craig


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Title: Re:Early Rotax Ignition theory
Post by: 1970TNT on December 12, 2014, 05:24:36 AM

WHAT ARE THE OHMS ON THE COIL? SHOULD BE AROUND 6200. SOUNDS LIKE YA MAY HAVE A BAD BUSHING ON THE POINTS AND THEY ARE GROUNDING TO THE STATOR PLATE SEEING AS THE CIRCUIT IS NOT OPENING.

QUESTION DID YA REPLACE THE SPARK PLUG WIRE AND CAP AND THEN CHECK TO SEE IF YA HAVE AN OPEN CIRCUIT?

YA BAD GROUND WILL DOO THE SAME THING. MAKE SURE YA PULL THE WIRE WHEN TESTING. I HAD ONE THAT DID THE SAME THING YOUR'S IS DOOING AND THE WIRE BROKE INSIDE THE INSULATION >:( >:(


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Title: Re:Early Rotax Ignition theory
Post by: craig on December 12, 2014, 08:11:45 PM

Thanx for response. Didnt mean to hijack thread, just seemed relevant to where I was in life. No wiring hooked up, motor in basement, flywheel not installed. Only 3 components to deal w/ I will find my ground.
Another question tho, when I get to the point of trying to time this where would i hook test lite? All videos show hooking test lead to wire going to xternal coil. I`m dealing w/ internal coil.Would I just time it at plug cap? Thanx for all the help, Craig



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