Bobine d'Allumage Tondeuse

Allumage tondeuse à gazon Briggs et Stratton

THEORIES OF OPERATION Ignition IGNITION A magneto in a sense consists of two simple circuits, one called a primary circuit and the other the secondary circuit. Both circuits have windings which surround the same iron core and the magnets in the flywheel or rotor act on both circuits. Current can be induced in each by changing the magnetism in or around the coils of the circuit. The primary circuit has relatively few turns of heavy wire and the circuit includes a set of breaker points and a condenser. The secondary circuit has a coil with many turns of lighter wire which are wound around the outside of the primary winding, and includes a spark plug. There are about 60 turns in the secondary to each turn in the primary. A permanent magnet is mounted in the flywheel or rotor. As the flywheel rotates, the magnet is brought into proximity with the coil and core. The Briggs & Stratton new ignition magneto system differs from ordinary magnetos in that the voltage produced is tailored to the needs of the engine. See Fig. 26. The magnet used in this new type is a ceramic which develops a very high magnetic strength in a very short distance. The length of this magnet is 3/8" as compared with the Alnico magnet length of 7/8"..

Figure 26. C:\AVIKIAN\Allumage tondeuse\bobine d'allumage tondeuse.doc

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Fig. 27 shows the flow of magnetism through the iron core of the coil as the magnet in the flywheel approaches the armature. The arrows indicate the direction of flow of the magnetic field. You will notice that there is no (or very little) magnetism flowing through the upper part of the core. This is because of the air gap at the top which causes a resistance. In this position our breaker points close

The flywheel continues to rotate to the position shown in Fig. 28. The magnetism continues to flow in the same direction and magnitude through the center of the core because of primary current. However, the magnetism flows in an opposite direction through the outer portion of the core and through the top air gap because of the change of flywheel position. Since the shunt air gap provides a path for the flux from the armature legs and the core, the required current flow through the primary circuit is low, assuring long breaker point life.

At this position our breaker points open, the current stops flowing in the primary circuit and therefore the electromagnetic effect ceases. The magnetism instantaneously changes from the flow shown in Fig. 28 to that shown in Fig. 29. Note the opposite direction of the arrows indicating a complete reversal of magnetism which has happened so fast that the flywheel magnet has not had a chance to move any noticeable amount.

The rapid change in magnetism produces 170 volts in the primary winding. A voltage is also induced in the secondary but it is in proportion to the turns ratio, i.e., 60 to 1 or 10,000 volts. This voltage is more than ample to fire across the spark plug electrodes. This rapid magnetism change is very short and therefore the flow of current across the spark plug gap is as long as necessary, but short enough to afford long electrode life. Thus we achieve our aims of full power plus long life and dependability.

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Figure 27.

Figure 28.

Figure 29.


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TIMING IS EVERYTHING - Basic Kart Ignition Explained PART I Few systems on your kart are more critical, or less understood, than the ignition system. For the most part, the ignition system does its job without much fuss. You hit the starter, or pull the recoil, and the engine fires up. If it doesn't, you put in a fresh sparkplug and everything is ok again. But a lot of us do not have a clue about how the ignition system really works. Over the next couple of months we're going to take a look at the ignition systems that are mounted on the Yamaha KT100S, the Briggs and Stratton, and on the current generation of Reed Valve engines. But to begin with, we need to have a basic understanding of how magneto ignition systems work. All kart engines use magneto ignition systems. Unlike the ignition system in your car or truck, a magneto doesn't require a battery as a basic power source to run the ignition system. Instead, it relies on principles of electricity and magnetism. When you wrap wire around an iron or steel core, and then change the magnetic field in that core, you induce an electrical voltage in the wire. Magnetically speaking, as the magnetic field of the magnet approaches the field that the wire creates around the metal core on which it is wound, a "potential" develops. The rate at which that potential develops determines it magnitude. It's how a generator works. The more coils of wire you pass through the magnetic field, the more voltage you induce in the wire. Spinning the coils on the armature of the generator past the magnets induces voltage in the armature. That's what powers your trailer lights at the track, or your power tools, or whatever. But it doesn't matter which element, the magnet or the coil of wire, moves and which is stationary. In your engine, the magnets are located in the flywheel or ignition rotor attached to the crankshaft. The coils of wire into which we're going to induce the voltage are located in the coil, which in this case is stationary. So unlike the example above, the magnetic field rotates past the wire, rather than the other way around. But the effect is the same. Moving the magnets past the wire induces voltage in the wire.

"Terrific" you say. "We've created voltage in the coil by moving the magnets past it." But I'm afraid I have some bad news for you. First of all, the voltage you've generated is way too puny to jump across the spark plug gap. As a matter of fact, it takes


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Allumage tondeuse à gazon Briggs et Stratton Secondary winding about 5000 volts to make the spark jump across a .030 inch gap. And even if you could somehow muster enough voltage to fire the plug, you've got only the vaguest idea of when that energy is going to have enough oomph to make the jump and start the combustion process. No, you need to be able to take the relatively small voltage that you've induced in the coil wires and then tell it exactly when you want it to fire the sparkplug. And secondly, you've got to jack up the voltage so it has the muscle to light off the fuel/air mixture in the cylinder.

Actually, the second part of this project is the easier of the two. Your coil actually has two coils of wire in it. (See Figure 1) The first one, called the "primary winding" is the wire in which the magnetic field induces the voltage. It is not even connected to the sparkplug wire. The plug wire is connected to the "secondary winding" of the coil, and the secondary winding has a lot more coils of wire in it than the primary, more "turns" they call it. It basically works like a transformer. The more turns, the more voltage for a given voltage. Putting 12 volts through the primary winding that has 100 turns and a secondary winding that has 200 turns will give you 24 volts in the secondary winding. Only in your ignition coil, the ratio of turns from the primary to the secondary is a lot more than 2 to 1. That is how the small voltage that the magneto generates in the primary winding gets stepped up enough to make the sparkplug fire. Like I said before, getting the voltage up enough to fire the plug is the easy part. Telling the coil just exactly when to unleash that energy is the tricky part. We'll be looking at the ignition systems used on the Briggs, the 100cc Yamaha, and the current crop of 2?cycle Reed engines. While each of these engines uses a slightly different technique to "trigger" the spark, they all rely on the same basic principle. As the magnets in the ignition rotor or flywheel approach the coil, the magnetic field begins to induce voltage in the primary winding. This voltage is grounded to the crankcase of the engine as a "dead short" circuit. The fact that the circuit is grounded, and therefore complete, and has the voltage potential we mentioned before, causes a current to flow. That ground may be through a wire coming out of the coil attached to the crankcase or it may be internal to the coil and not be visible. At some pre-determined point (remember, each engine type does this differently) this short circuit opens. That means the voltage can't ground out anymore. Instead, it "bounces" back through the coil, ti energizing the secondary winding, and that voltage grounds itself out by jumping the spark plug gap to ground on the engine. Think of it This way; if you've ever closed a water faucet real fast, you sometimes get


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Allumage tondeuse à gazon Briggs et Stratton a "shock wave" in the water pipes. They call it "water hammer" and it makes a noise like someone banging once on the water pipe. Sometimes it happens when the washing machine is running and one of its valves closes real fast. Anyway, the same thing can happen, more or less, to electrical current flowing through an inductor. If the current flowing to the ground in the primary circuit of the ignition system is suddenly stopped because the circuit is broken, the current "bounces" back. In electrical circles it's called "inductive fly-back" and it's what creates the voltage spike that the the coil then steps up to enough voltage to jump the sparkplug gap.

Certainly the location of the magnets in the flywheel, relative to the location of the coil affects when, relative to the position of the piston in the cylinder, the voltage rises and gets things going. But exactly when the primary circuit opens and initiates the process that actually fires the sparkplug involves several other, more sophisticated factors. Next we will look at those for each type of engine and why they are so important. Remember, when the magnets in the rotor or flywheel on the crankshaft rotate past the windings of wire in the coil, they induce a voltage in those windings. But that little voltage doesn't have enough voltage to jump the gap on the sparkplug. We have to "muscle it up" to about 5000 volts or so to be able to consistently ignite the compressed fuel/air mixture in the cylinder. That is where the secondary winding, with a lot more "turns" of wire, comes in. Like a transformer, it boosts up the voltage to where we need it to get the job done. Creating the potential and jacking up the voltage are the easy parts of this project. Getting the spark to happen at precisely the right moment, relative to the piston's position in the cylinder, is what makes all the difference. There are a variety of mechanisms to control when this happens. Remember, for a good part of the time that the magnets are moving past the coil and inducing voltage in the windings, that voltage is shorted out directly to the crankcase. But at some point, that short circuit has to be interrupted, causing the voltage to "bounce back" through the coil, energize the secondary winding, and fire the plug. In the older engines, like older Briggs', McCulloch 2?strokes, and the like, this job was handled mechanically. A set of "points", little spring-loaded contacts, actually opened and closed, actuated by a cam lobe located on the crank or the hub of the flywheel. The points stayed closed for most of the crankshaft rotation and the current flowed through them and grounded out to the crankcase. When the cam lobe came around and the points opened, the short circuit was broken and the sparkplug firing process began.


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Allumage tondeuse à gazon Briggs et Stratton Today's engines, whether 2 cycle or 4 cycle, have moved past this rather primitive mechanical operation to electronic ignition. These solid-state systems are more durable and reliable than the old points systems, but more importantly, they give pinpoint control of what engineers call the "spark event". The Yamaha uses a special type of magneto system with the "trigger" mechanism housed in that little gold box that attaches to the outside of the engine. For as expensive as that X%*?@ little thing is (Yamaha calls it the TCI module), it is surprisingly simple inside. The primary current flows through the wire coming out of the coil, into the TCI module, and grounds through the case of the TCI. (See Figure 2) That is why it is so important to be sure you have a good connection between the TCI and the engine crankcase. Be sure it is mounted securely, either directly on the engine or to something connected to the engine. The TCI contains a small circuit board with a few resistors and capacitors, and a good-sized transistor. The transistor is the heart of the system and it, in effect, "measures" the amount of current flowing through it. As the rotor moves past the coil, the voltage builds, reaching a peak when the magnets are just about centered on the coil. Just before this peak, the transistor in the TCI reacts. Think of it as flipping an "electronic switch". The circuit opens and, with no place to go, the voltage bounces back. That voltage spike in the primary windings of the coil energizes the secondary windings and that high voltage then goes in search of someplace to go. The path of least resistance is across the sparkplug gap to ground on the cylinder head. Bingo! The plug fires and so does your engine. The only variable that affects when the spark occurs is when the rotor/coil combination generates the correct current to open the transistor in the TCI. There use to be a significant variance between the older TCI boxes, measured as current required to open the transistor. But, curiously, these differences did not translate into any measurable timing or performance differences. The newer TCI boxes, standard with Yamahas and PRDs, have virtually no variance in current value required. Since the tech rules dictate that the ignition timing key and rotor are fixed, this means that, regardless of the TO value, all Yamaha KT 100s have about the same ignition timing, measured in degrees of crank rotation before top dead center.

Again, the principle is pretty much the same. The rotating magnet on the ignition rotors moves its magnetic field through the field surrounding the fixed coil. That induces a potential voltage in the primary windings of the coil. That potential causes a low voltage current to flow from the coil via the primary wire, to the TCI, and through the TCI to ground against the engine. When the current reaches a pre-determined value engineered into the transistor in the TCI box, that transistor opens and the


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Allumage tondeuse à gazon Briggs et Stratton current suddenly stops flowing. "flyback" effect causes a voltage spike to occur in the primary winding and that energizes the secondary winding. With many many times more wire turns in the secondary than the primary, the voltage potential induced in the secondary winding is much higher. High enough, in fact, to jump the sparkplug gap to ground itself, causing the spark, which is what we really wanted all along.

The critical factors are: • •

Solid connections between the coil primary wire and the TCI box. Solid connection between the TCI box and the engine casting.

No shorts or other interruptions to divert the current from the TCI box. This last point is important because it is how you hook up a kill switch to the Yamaha, if you wish to. A simple wire from the connection between the primary wire and the TCI, running to a switch does the trick. Just hook the other side of the switch to a wire that is grounded and, when the switch is flipped, the current from the primary wire will take the path of least resistance and bypass the TCI box and go through the switch to ground. Bingo! No current to the TCI box, no spark. Most of the Reed Valve 2 cycles in use today, including Gearbox engines, using some variation of the CDI systems first developed for Italian kart engines in the 70s. CDI stands for "Capacitive Discharge Ignition. Once again we have a rotating magnet and a fixed coil or "stator". In contrast to the Yamaha system, however, the control module is housed in the stator with the primary windings, and the coil is external. Those primary windings called the Figure 3 "charge coil" in the stator are mirrored in the external "ignition" coil, where they share space with the secondary windings. But also in the stator is the "pickup" or "pulser" coil. (See Figure 3) So, as the magnets in the rotor move past the pickups in the stator, they induce voltage in the primary windings of the charge coil in the stator. When the control circuitry in the stator, triggered by a signal from the pulser coil, disables the current's path to ground, that current flows through the wires connecting the stator to the coil where it flows through matching windings. These, in turn, power up the secondary windings and the voltage is discharged across the sparkplug gap. There are important differences to note between these systems, whether they be Selecta, ltalsystems, or whoever. Some also include an external Capacitive Discharge module in addition to the external ignition coil. But in each case the statorto-coil connection consists of at least two wires; a primary and a ground. This means that you don't have to worry about mounting the coil on the engine itself. As long as the wires reach, you can mount the coil wherever it is convenient. Often this means mounting it in a location more protected or less subject to vibration. The second difference, and the one that can make an important performance difference, is that using three sets of windings means that the voltage can be "stepped up" a bit more than the two set configuration in the Yamaha ignition. That can yield a hotter spark and more efficient and complete combustion.


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Allumage tondeuse à gazon Briggs et Stratton TIMING IS EVERYTHING - Basic Kart Ignition Explained PART 2 Last month we began looking at the basic principles that make the ignition system on your engine work. You'll recall that we noted that when the magnets in the rotor or flywheel on the crankshaft rotate past the windings of wire in the coil, they induce a voltage in those windings. But that "potential" doesn't have enough voltage to jump the gap on the sparkplug. We have to "muscle it up" to about 5000 volts or so to be able to consistently ignite the compressed fuel/air mixture in the cylinder. That's where the secondary winding, with a lot more "turns" of wire comes in. Like a transformer, it boosts up the voltage to where we need it to get the job done. We also looked at the magneto ignition systems used in the Yamaha KT l00S engine, and took a rather general look at the Capacitive Discharge Ignition systems used on most of the Reed Valve 2 cycles today. The largest share of the 80cc and 125cc engines also use some variant of this CDI ignition. Now let's look at the venerable Briggs & Stratton and its "Magnetron" ignitions system. It seemed impossible to believe when Briggs told us we could yank out the points and condenser that had served us to well for years and plug that little box in their place. Well, not only does it work, but also it works better than the old points ever did. Here's how: The magnetron coil actually contains 3 separate coils of windings. In addition to the primary and secondary windings, there is a 3rd winding, called the "trigger" coil." When the leading edge of the magnets in the flywheel approach the coil, the magnetic field surrounding those magnets generates a small voltage (about 1 volt) that powers a solid-state switching device called a Darlington Transistor, turning the transistor "on." That "on" mode completes the circuit on the primary winding and a current of about 3 amps flows to ground on the crankcase (Figure 2). As the flywheel continues to rotate that current builds the magnetic field in the secondary winding.

When the trailing edge of the magnets in the spark plug gap. the flywheel reaches the trigger coil, a second small current is induced in the trigger coil and that current tells the transistor to "turn off", effectively breaking the circuit of the primary winding to ground (Figure 3). As we discussed last month, the sudden break causes the magnetic field to collapse. Since current (amperage) and voltage are inversely proportional, suddenly stopping the current flow causes the voltage to "spike" as it compensates. It's that spike that builds the voltage required in the secondary winding to jump the sparkplug gap and ignite the mixture. By the way, in the Magnetron coil, the primary winding has 74 turns of wire and the secondary has 4400. That's a 59.5:1 ratio and that helps insure that there's enough voltage to get the job done. All this, the initial current in the trigger coil, the buildup of potential, the second trigger coil


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Allumage tondeuse à gazon Briggs et Stratton pulse, and the collapsing field with the resulting voltage spike and spark, in on about 10 degrees of crank rotation. That means that at 6000 RPM the whole thing, start to finish, only takes about 0.00027 seconds. Pretty amazing.

This simple, but very effective, spark control system even advances the ignition timing, causing the plug to fire earlier (in degrees of crank rotation) as RPMs go up. You may recall from the part on of this series (NKN January,2000) that "the rate at which (the) potential develops determines its magnitude." In other words, the faster you spin the magnet on the flywheel past the windings in the coil, the greater the potential voltage that is developed. Since it only takes about 1.0 volts from the trigger coil to turn the Darlington transistor on (and off), the faster (in terms of crank rotation degrees) the potential builds to the required 1.0 volts, the earlier the induced current in the primary side of the coil will begin to build. And likewise, the earlier the second trigger coil potential reaches 1.0 volts, the earlier it will turn the transistor off and trigger the spark process. Take a look at Figure 3 to see how this advances the timing. I know this sounds like a lot of engineering gobbledygook. But what it means to you as a racer is, the faster you turn the engine, the earlier the spark occurs in the rotation of the crank. "Advancing" the spark is something that engine tuners from karts to NASCAR have played with for years. Most big-car ignition systems have had built-in ignition advance for decades. In more recent years, old mechanical systems to change the timing in the distributor have been replaced with sophisticated electronic engine control systems. The Briggs "Magnetron" system does exactly that, cleanly, efficiently, and reliably. There are only a few factors in the ignition system that the racer can adjust. Some are relatively simple and tuning with them is common practice. Others require sophisticated machining and fabrication work. Still others are just a matter of swapping components. In any case, you can make some changes that will have a dramatic effect on how your engine performs by working with the ignition system.


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Allumage tondeuse à gazon Briggs et Stratton Let's begin by seeing what happens when we start fiddling with the Briggs. Probably the most commonly used ignition-tuning tool on the Briggs & Stratton 4stroke is the offset flywheel key. Every kart shop carries them and the savvy Briggs racer carries a selection to adjust the engine to the track. Basically, these little machined pieces replace the stock Briggs 1/8" X 1/8" flywheel key. Although they are sometimes marked in thousandths of an inch offset, more commonly they are identified by degrees of offset. In other words, how many degrees of crank rotation do they move the flywheel from the stock position. Most engine builders building methanol-burning Stock-class engines usually send their engines out the door with 5 degrees or so of offset. That amount of advance provides a reasonably safe performance increase without too many headaches for the less experienced or adventurous tuner. Part of that improved performance comes from starting the combustion process itself earlier and thus optimizing the point at which the combustion chamber pressure reaches its peak. Another part comes from the fact that the methanol/air mixture burns somewhat more slowly than a gasoline/air mixture for which the engine was designed. So you gas-classers out there take note; 2 or 3 degrees is a better place for you to start.

One thing to be careful of, silly as it seems, is that you offset the key the correct direction. You want the sparkplug to fire earlier in the crank rotation, when the piston is farther down in the cylinder on its way up. That means that, when viewed from the flywheel side, with the starter or starter nut removed, along with the washer, the keyway slot in the flywheel should be closer to top-dead-center than the keyway slot in the crankshaft, as the keyway rotates up toward the coil (Figure 4). Ok out there, l see you rolling your eyes. But 1 have seen engines come in with the flywheels offset the wrong way. And believe me, retarding the spark 5 degrees or so won't do anything to help your performance Anyway, what happens when you move up or down from that 5 degree starting point? Offsetting the flywheel even more helps the engine achieve more complete combustion at higher RPMs. Starting the fire earlier, you know. But more combustion time in the cylinder means more heat absorbed in the block and head too. You'll gain top end but you'll probably need a bigger jet to keep the heat below the 380?390 degree upper limit. If you've blocked off part of the air intake on the fan shroud you


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Allumage tondeuse à gazon Briggs et Stratton may want to open that back up a bit. Most tuners aren't afraid to go up to 7 or 8 degrees, but few venture beyond there. Unfortunately, that top-end increase comes at the expense of low end. And on track where getting off the slow corners is the difference between winning and losing, that's a poor trade. In cases like that, an experienced tuner reduce the flywheel offset to 4 or even 3 degrees to improve low RPM performance. Since the fuel/air mixture always burns, more or less, at the same speed, if you need the engine to be at its best at lower RPMs too much advance can move the peak combustion pressure back so early in the crank rotation that it actually resists the momentum of the crank and flywheel and hurts performance. Of course, to get the optimal fuel mixture at these lower RPMs, and to help get the heat up into the desired range, you may need a smaller jet if yon reduce the offset in the ignition. Closing off some of those air intakes in the shroud may help get the heat up too.

There's only one problem with offsetting the flywheel key to get the timing where you want it. The folks at the Briggs factory took great pains to balance the flywheel/crankshaft/piston/rod combination for smoother running and better performance. Offsetting that key upsets all that carefully engineered balance, and that hurts performance. So are you shack having to choose one factor over another? Not necessarily. Next month we'll look at how you can get the timing you want without giving up too much in the balance area. Then we'll conclude this look at ignition systems by examining what other adjustments and tuning features each system offers and what affect those adjustments can make. See you then.


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Allumage tondeuse à gazon Briggs et Stratton Ignition System Theory and Testing

The 1980's ushered in the magic black box, Magnetron™ ignition coil, Briggs & Stratton's first truly electronic ignition system. Breaker point ignition systems for most small air-cooled engine manufacturers have totally disappeared over the last 20 years. In a point style system, the flywheel magnets rotate past the legs of the ignition armature. The armature itself is made up of two separate windings of copper wire - the primary and secondary - one wound on top of the other. Basic physics tells us that when a magnetic field (flywheel magnet) cuts through (moves past) a conductor (copper wire), a flow of electrons (electricity) is created. However, electron flow only occurs when we have a complete circuit. This means that the points must be closed. We're also told that the faster the movement between the field and the conductor, the greater the output. Remember science class in grade school? At one point in your school career, an enterprising teacher wrapped a length of copper wire around a nail and hooked the wire ends to a dry cell battery. A handful of paper clips was instantly attracted to the nail and fell away when the battery was disconnected. Electron flow through a conductor then, causes a magnetic field. When the points close, electron flow causes a magnetic field to be created around the primary. This field also envelopes the secondary. The points now open, break the circuit and collapse the field through the primary. The field caving in on itself is movement just like the rotating magnetic field of the flywheel. This movement is at speeds much greater than the flywheel could spin - near the speed of light. The rapidly collapsing field tears through the secondary winding which has sixty turns of wire for every one turn of the primary effectively generating 60 times the voltage created in the primary. The sum total of this is that the secondary winding can create up to 25,000 volts in some systems, which is used to jump the air gap of the spark plug and ignite the fuel/air mixture in the combustion chamber

Now for the black box. Magnetron™ solid state ignition systems, in essence, replace the mechanical breaker points with a transistor. That is, we replace a mechanical switch with an electronic one. No moving parts, no arcing, no adjustments and solid state reliability. Now that we've got an idea of how it all works, let's look at the meat and potatoes of what is required to create a good spark.

Flywheel: The flywheel magnet must generate a sufficient magnetic field to start the chain of events in motion. A fair test is to hold the flywheel on edge with the magnet facing up. Place the blade of a 10" #3 (1/4") straight blade screwdriver against the magnet. Release the screwdriver. The magnet should have enough strength to hold the screwdriver straight out. If we pass this test, assume the magnet is OK.


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Allumage tondeuse à gazon Briggs et Stratton Rotational speed:

Remember speed is a factor. The engine must be pulled over at a minimum speed of 250 RPM before the coil will even think about firing. Thick oil on a winter day or a heavy parasitic load may cause problems. Customers come into play here as well. Shorter or elderly individuals may not have the leverage or strength required to reach the RPM required to activate the Magnetron's electronics. Spark Plug: The spark plug is a major element of the equation. A new spark plug may require around 10,000 volts to jump a .030" gap when the engine is cold. This drops to just 4,000 when the engine is hot as electrons are more easily emitted from a hot surface. That's one of the reasons the old vacuum tubes in radios had to warm up before the radio would work. Electrons are also more easily emitted from a sharp edge than a round one. A spark plug begins to require more and more voltage as the edge of the center electrode becomes less defined. And finally, an internal short or carbon/oil fouled plug simply shunts the high voltage burst straight to ground with no or insufficient spark. Ignition Coil: The ignition coil is probably the easiest thing to check and therefore the first thing to check when embarking upon ignition system troubleshooting. Install the 19368 spark tester between the high-tension lead and a good engine ground. Spin the engine over (at least 250 RPM) and watch for spark in the tester window. As simple as it seems, this is a fairly comprehensive test. The tester electrode gap is .166" wide. Those wise in the way of electrons have calculated that it takes around 13,000 volts to jump this gap. We need 10,000 to jump the gap on a cold spark plug. Add it all up and we have voltage to spare. As coil temperature can aggravate minor coil imperfections that normally wouldn't be a factor, the same test can be done on a warm engine. Engine quits while running? Hook the tester up in line with the spark plug and start the engine. When the engine quits, monitor the window. If spark is present, the problem is not in your ignition coil. By the way, this test stresses the coil well beyond the demand it would see in operation. Think about it. We're asking the coil to build enough voltage to jump TWO gaps - the tester as well as the plug. If your engine starts and runs OK cold and hot, you've got a healthy ignition coil. One additional test you can perform. Check the impedance (resistance) of the secondary circuit at room temperature. Hook an ohmmeter test lead to the spark plug terminal of the high-tension lead and another to the lamination stack (ground). Your resistance reading should range between 2,500 and 5,000 ohms. If infinite (no continuity), an internal open circuit exists. Replace the coil. If infinite and the engine runs, your problem is an internal break of the high tension lead, a poor attachment of the spark plug terminal or improper mating of the high tension lead to the coil. A pin within the coil body skewers the lead. If the pin does not contact the wire core, there will be no continuity. The coil will often have enough available voltage to jump the C:\AVIKIAN\Allumage tondeuse\bobine d'allumage tondeuse.doc

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Allumage tondeuse à gazon Briggs et Stratton gap, so you see spark. The internal arcing that occurs within the high-tension lead will eventually create enough resistance that ignition system performance will suffer. If your resistance reading is much lower than 2,500 ohms, an internal short exists. Replace the coil. Now, how about some of those old wives tales that just aren't true. • •

•

•

Rust on the flywheel magnets causes a loss of spark. Not true. A magnetic field does not care about rust. It has no effect on it. A bright blue spark is best. A yellow/orange spark signifies weak ignition. Not true. Spark color determines virtually nothing. The hottest spark is ultraviolet which we can't see. Blue spark is cold in comparison to ultra-violet. Orange and yellow come from particles of sodium in the air ionizing in the high energy of the spark gap. Laying the spark plug against the block and pulling the engine over can adequately test ignition coil output. Not true. The ignition coil will only generate enough output to jump the gap of the plug. When under compression, the plug requires twice the voltage to fire. This check is not an accurate test of the coil and can be misleading. An armature air gap that is too wide will prevent spark. Not true. Well, sort of not true. Briggs & Stratton air gaps cannot be made too wide to prevent spark providing the coil is healthy and the engine is spun over fast enough. A wide air gap, say .030" will ever so slightly retard the ignition timing as the magnetic field takes longer to build within the coil windings.

Ignition coils, particularly Magnetron™ coils, rarely fail. If one is suspect, perform the outlined checks exactly as mentioned. MOST IMPORTANT: Be sure to isolate the coil from the equipment wiring harness as well as the engine's wiring harness. That's right, unhook the ignition grounding lead from the coil itself and use the spark tester. Many a technician is fooled into replacing a good coil because the coil grounding lead was shorting out against a piece of sheet metal. DO NOT attach the tester to the spark plug for this test. The engine may start. Without the grounding lead installed, you won't be able to turn it off. If the coil is properly grounded to the engine block, engine speed is at least 250 RPM and the flywheel magnets are OK, there should be spark present in the window of the tester. If not, repeat the test double checking your procedure. Still no spark? Then and only then, replace the coil. A final bit of trivia - All Magnetron™ Ignition coils have the manufacturing date code cast into the coil body. The coil manufacturing date will usually be within a month of the engine's date code. That's an easy way for you to tell if the coil has been changed before. We use this information to match returned parts to the engine noted on a warranty claim as well as for internal tracking


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Bobine d'Allumage Tondeuse

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