Ignition systems always seem to create difficulties for those who tune high-output, two-stroke engines, and for good reason: First, the two-stroke engine's ignition system marches double-time, as compared with a four-stroke operating at the same crank speed, which not only doubles the rate of wear at the breaker points but also raises the heat input to the spark plugs. Secondly, the plugs are subjected to a fog of oil between firings that bakes into a layer of semi-conductive material over the plug insulator. Therefore, the tuner faces a situation in which the ignition system's ability to generate sparks deteriorates rapidly, while the voltage required to actually fire the plugs is on the increase. Difficulties inevitably occur, but these can be resolved - usually without resorting to draconian measures - if there is a clear understanding of ignition fundamentals.
Central to the function of all ignition systems is the high-ratio transformer we call a "coil", which comprises two sets of windings around a laminated, soft-iron core. Induction coils of this type employ the mutual influence of magnetism and electrical current: pass a current through a coil of wire and you create a magnetic field; sweep the magnetic field through a coil and you will induce a current in the wire. So in the common ignition coil, you will find primary windings to create an electromagnetic field, and secondary windings in which an electrical potential is induced. The arrangement constitutes a kind of electrical transmission, in which, for example, 12-volts are applied to about 400 turns of primary windings and amplified by the approximately-15,000 turns of wire in the secondary windings (a step-up ratio of 375:l) to the 20,000-plus volts needed to produce a spark at the plug. With 100-percent efficiency, the coil would increase the primary voltage to 45,000 volts in the secondary windings (375 x 12 = 45,000) but that is a practical impossibility. Other practical difficulties include the limitation on the amount of current the primary windings will accept without melting, and the voltage that can be. induced on the secondary side before the resistance of the internal insulation is overcome and short-circuit sparking occurs inside the coil. Also, there are limits to the electrical load the ignition system's contact-points will carry, and extremely high secondary voltages can cause rapid electrical erosion of the spark plug's electrode.
All of the presently-used ignition systems feature the high-ratio transformer, or coil, just described; there are several ways of triggering that coil into action and these can have a considerable effect on ignition performance. The system most commonly employed on touring engines is one in which current is drawn from a battery, passes through the primary windings, and then through an engine-driven switch (the "points") to ground –which completes the circuit, as the battery has one terminal connected to the motorcycle's frame, or ground. It should be understood that the ignition points are normally closed, and that a spark is obtained when the points are bumped open by the contact-breaker cam. There is a voltage rise in the secondary windings when the points close and current starts to flow through the primary side of the coil, but due to a phenomenon called, "impedance", the magnetic field builds too slowly to induce maximum voltage. Only when the points open, and the primary current disappears, does the field move quickly enough - in collapsing - to induce ignition voltage. Because there are so few primary-winding turns, compared with the secondaries, the field collapse creates a very much lower voltage there, but it would be high enough to cause an electrical arc to form between the point contacts, which have only barely opened, except for the action of the condenser. This last item acts as a kind of electrical reservoir, and it receives the induced primary current and prevents voltage from rising high enough to form an arc across the opening points. Should the condenser fail, an arc will be established, and the effect is rather like having the points closed, with the current in the primary windings reaching a level sufficient to choke back the magnetic field's collapse and thus drastically reduce the voltage in the secondary side of the coil.
Viewed strictly as regards its spark-producing ability, the old battery-and-coil ignition system is limited, but is not nearly as bad as present-day enthusiasm for the more elaborate alternatives may make it appear. It requires that you carry a battery on the motorcycle, and that battery must be rubber-mounted to prevent vibration from shaking its plates apart, but a battery large enough to energize the ignition system for the duration of a race is little heavier than a magneto and, moreover, does not borrow from the engine's power output. You must, of course, keep the battery on a low-rate charger between race meetings to prevent sulfation of its plates, and if you are the forgetful type that may be counted a short-coming. Still, the battery-and-coil system's single serious deficiency is simply that it does not produce sparks quickly enough. By that, I do not mean that it is slow in terms of the total number of sparks it will yield in a given time period (it is capable of making at least 10,000 sparks per minute), but simply that the voltage to produce a single spark builds rather slowly. That characteristic is most unfortunate when combined with the perpetually semi-fouled condition of the two-stroke engine’s spark plugs, for if spark voltage rises too slowly, the electrical potential in the coil's secondary windings will leak away across the deposits on the plug insulator and voltage will not climb to the level required to ionize the gases in the gap between the electrode and the ground wire. In that instance, no spark occurs, even though the basic system may be capable of delivering voltages of 30kv (30 kilovolts, or 30,000-volts), which certainly would be enough to produce ignition with a clean plug. Indeed, touring engines running on relatively lean mixtures, with only minimal amounts of oil passing through, and with "hot" plugs perform very satisfactorily when sparked by the humble battery-and-coil system. Racing engines, however, must be fed rich mixtures, lots of oil, and be fitted with decidedly cold plugs – which means that they often will run without the slightest sign of distress on wide-open throttle (a condition that keeps the plugs burned clean) being supplied sparks from a battery-and-coil system, but lapse into a fit of misfiring the first time the throttle is closed.
A magneto is the most commonly-applied alternative to the battery-and-coil ignition system. Many people assume that the single virtue of the magneto is that it eliminates any need for carrying a battery on the motorcycle, but in the context of racing that aspect is merely a minor convenience. A far more important justification for magneto ignition is that it provides a far more rapid voltage rise than a battery-and-coil system. Racing plugs have a very short insulator nose, which means that they also have poor resistance to "wet" fouling- the kind of spark-voltage short-circuiting that occurs when the plug nose is coated with soot and raw gasoline. This mixture isn't what you'd call a good conductor of electricity, but it is good enough to drain off the slow-rise voltage delivered by battery-and-coil ignition system. The critical voltage rise time seems to be about 45 microseconds: if the electrical potential across the spark gap reaches 15kv in 40 microseconds or less, misfires due to wet fouling rarely occur. And a good magneto delivers maximum voltage to the plug (a maximum that may easily exceed 30kv) in only 35-40 microseconds after the system is triggered.
The explanation for this faster rise-rate is found in the manner in which magnetos make sparks: Energy is supplied by an engine-driven, rotating magnet, which sweeps its field through a coil, or coils, to generate the operating voltage. In some magnetos all of the coils are contained within one housing, but most motorcycles have "energy transfer" magnetos (originally developed for high-altitude aircraft engines). These have low-tension generating coils connected in parallel with the breaker points and the spark coil. The points, and the phasing of the rotating magnets relative to the generating coils, are timed together, for the points must open when the magnetic flux is strongest. With the points closed, and the magneto rotor swinging into position next to the generating coil, the rotor's magnetic field sweeps through the coil and induces a strong current. This current is, in effect, short-circuited through the points, so that the wire in the coil passes the electricity around in a continuous loop, and that flow of current makes the coil into an electromagnet with a field polarity opposite that of the rotor. So you have two magnetic fields pushing against each other, and that condition persists until the points open. Then the current in the generating coil is interrupted, and its magnetic field collapses, which permits the rotor's field - at that moment forced back like a spring - to snap through the coil. The extremely rapid flux change induces an extraordinarily strong voltage surge in the generating coil, and this is communicated to the primary windings of the spark coil-which responds by producing enough voltage to fire the spark plug. Substantially the same process occurs in the traditional magneto, in which the generating coil is also the spark coil's primary winding, and is overlayed by the high-tension windings.
The principal reasons for using the E.T. magneto are that A) the spark coil is removed from direct proximity with the engine, where it frequently becomes too hot for its own survival, and B) it allows you to keep the length of the high-tension spark lead to a minimum. This last is especially important, for while electrical resistance is unimportant here, ignition leads have capacitance and inductance properties that adversely effect voltage delivered to the spark plug and should therefore be kept as short as possible. By and large, magnetos arc very reliable, but when trouble occurs, look for either pitted (or dirty) points, a faulty condenser, or - more rarely - a weak coil. Especially, make every effort to keep the points clean, smooth, and adjusted. The contact surfaces have to carry a considerable electrical load, and if they become burned, or filmed with oil, spark voltage drops very sharply. After all, the voltage induced in the spark coil is in the final analysis a product of the flux change down at the field coil, which is in turn largely a function of the current strength in the field coil just prior to the points' opening. Oil films and pitting increase the resistance across the point surfaces and reduce the current strength. Condensers fail mostly because the condenser usually is tucked away in a housing on the engine next to the points. There, they become overheated, their internal insulation softens, and short-circuiting follows. In my opinion, a racing engine's condenser (s) should always be mounted externally, even though that arrangement may not be visually tidy. I prefer placing the condensers next to the, spark coils. All of the magneto's elements are connected as a parallel circuit, so you can simply attach the condenser leads to the spark coil's input terminal. I might mention here, too, that it is unlikely' that you will find a condenser too large to give good results on the magneto. I have used the big Mallory condensers in many applications and find that there often is an improvement in ignition performance with the added capacitance. Why? Probably because the larger capacity reduces even further the slight arcing at the points, and gives a cleaner termination of the field coil current.
Sheer mismanagement probably is the most potent source of all problems encountered with magnetos. As noted earlier, the magneto's performance is strongly dependent on having the points break right at the moment of maximum flux, which means that when you are timing the points to deliver a spark at the correct moment in terms of crankshaft rotation, you must also "time" the magneto’s rotor and stator. Usually there will be an indexing mark on the rotor to be aligned with a similar mark on the stator; otherwise you will need a manufacturer’s manual dealing with the magneto’s construction to give you instructions as regards the orientation of rotor and stator. You can adapt a magneto from one engine to work on another, but the conversion isn't always easy: One difficulty arises because the air-gap between the rotor and stator (the stator is the laminated core for the generating coil) is very small, and while the rotor will center more or less automatically on the crankshaft unless the remachining of tapers is incredibly sloppy, getting the stator mounting concentric with the rotor can be quite a job. Whatever else you do, just don't compensate for misalignment by trimming away at the stator's poles, as a very small increase in the air gap will cause a marked sagging of the magneto's output. In fact, you shouldn't even drill the stator's mounting holes oversized when making the installation. As it happens, the magnetic field shifts that induce an electrical current in the magneto's windings do the same job in the stator, and there is the same creation of an opposing magnetic field, only this one cannot be controlled by the contact breaker points. For that reason the stator - which could more conveniently be made in one piece - is assembled from a number of thin laminations. With the induced current divided and confined, the magnetic effects are minimized, but if you drill through the stator laminations, or grind away at its pole surfaces, you break through the coatings of varnish that insulate between the laminations, establish contact, and convert the stator into another electromagnet. Of course, the spark coil laminations are similarly insulated, for similar reasons, and cutting through them also is poor practice. These things are commonly done, out of ignorance of the consequences -which usually are not so severe as to be responsible for outright misfiring but should be avoided in the interest of getting the best performance possible from any given ignition system. Finally, I would caution against simply assuming that bolting a coil to a motorcycle's frame automatically insures a proper ground. On many occasions I have been asked to help solve a persistent and mysterious ignition problem, only to find that the single difficulty was an incomplete circuit: the coil had been bolted to the frame with a fine disregard for the insulating properties of paint. Persistent coil failure is another complaint frequently heard, particularly with reference to the old internal-coil magneto. Overheating of the coil usually is at fault here, but you should be aware that those who made the coil planned for the temperatures involved and internal short-circuiting usually will not develop unless there is an electrical overload. The overload comes when a spark lead is lost, and the voltage from the coil no longer has a path to ground via the spark plug. If conditions are such that the electrical potential between the plug's electrode and ground wire ionizes the gases there when coil voltage is at 15kv, then that is as high as the voltage will go, because all the coil's energy will discharge through the plug and most of the discharge period will be at a voltage very much lower than that present ns the spark was formed. But if the spark lead is disconnected for any reason, the coil's internal voltage, will climb extremely high, and may become high enough to force through the insulation. When that occurs, a permanent path to ground is established and much of every successive voltage surge will burn its way across the same path. The result is a sharp loss of voltage at the plug, and the urgent need for a replacement coil. Heat will soften the insulation inside the coil and make short-circuiting easier, but more often than not the trouble would have been avoided had the tuner been careful to keep the leads in place - and to ground them in some manner when motoring the engine (by pushing the bike) with the spark plugs removed.
"Transistorized" ignitions are becoming more popular, and in some circles are regarded as the sovereign remedy for all ignition ailments. They are something other than their reputation suggests. Ordinary battery-and-coil systems can be transistorized, by using the points to valve "trigger" current to a transistor - which actually carries the electrical load. This kind of system offers no particular advantage for most motorcycle ignition systems, as its principal virtue is that it will handle the very high primary currents encountered in multi-cylinder automobile engines. Also, the points will last almost indefinitely, as they are under only a micro-amp load. The basic transistorized ignition system can be embellished to include a magnetic triggering device that replaces the mechanical contact breaker points entirely, and the advantage here is that all problems with point-float are eliminated and the sparks may be delivered with great accuracy. A disadvantage is that the circuitry becomes rather complicated, and an external power source (a battery) is required. The transistorized battery-and-coil ignition system with magnetic triggering is capable of delivering an extraordinary number of sparks per minute, but the character of individual sparks remains the same as is obtained with conventional points and I am not persuaded that this type system offers any advantages in terms of high-speed, high-output two-stroke engines.
There are advantages with other types of transistorized ignitions, if other of the system's elements are properly arranged. As in the case of the capacitor-discharge system, which can be made to be battery operated, or as a magneto-type device. Either way, there is a fairly conventional spark coil, and the spark is produced by the discharge of the capacitor's electrical contents into the coil's primary windings. Usually, the capacitor will be charged to about 400 volts, which is enough to produce a. more-than-adequate voltage at the coil's output lead. But the capacitor-discharge system's best feature is that it is tremendously fast, faster than even the conventional magneto: Something in the order of 50-100 microseconds are required for a single electrical pulse from a conventional battery-and-coil to reach sparking strength; a magneto requires 35-40 microseconds (which is just below the wet-fouling level of 45 microseconds); but a good capacitor-discharge system takes only a couple of microseconds in climbing to full voltage, which means that it is so far below the wet-fouling level that it may be considered virtually immune to that disorder. Moreover, its spark is delivered so rapidly that firing will occur even when the plug is very badly fouled with oil or anything else. The single disadvantage of the capacitor-discharge ignition system is that it can deliver this terrific spark so briefly that ignition of the air / fuel mixture in the combustion chamber does not follow. Magnetos, for instance, keep their sparks going for about 0.5-1.0 millisecond, while the capacitor-discharge system's spark may last less than a tenth of that time. It has become common practice to add a resistor in the circuit to prevent the capacitor from dumping too fast and delivering a too-brief spark to the plug, but there still are systems being made without this feature - which is to be regretted, as cold-start problems will occur even with everything else in order, including a fresh, clean plug. A too-brief spark is also ,likely to be unable to fire the kind of rich mixtures needed by two-stroke racing engines.
Sports / touring motorcycles, like the 750cc, 3-cylinder Kawasaki usually have a battery powering their capacitor-discharge ignition system. The battery's 12-volt potential is multiplied to the required 400-volts by a special oscillating circuit and transformer, and the capacitors dumped into the spark coils via transistors of the type called SCR (silicon controlled rectifiers). The SCRs are actuated by separate circuits, which are in turn controlled by magnetic trigger. Conventional points will also work here, but most manufacturers seem to feel that as long as they have to resort to solid-state electronics, they might as well go all the way and dispense with the breaker-points. A more suitable system, for racing, is one in which the capacitor (or condenser) is charged directly from a magneto-type rotor/stator arrangement with an appropriate generating coil. By inserting a rectifier-type transistor between the generating coil and the capacitor, voltage is trapped in the latter, to be used in activating the spark coil. With a little jiggling of circuitry, it is possible to use the same rotating magnets to charge the capacitor, and trigger an SCR into releasing the charge to the coil, and this is the arrangement you would find inside the encapsulated wiring of most current "CD magnetos" if you could get them apart. There isn't much I can tell you about these ignition systems except that when they work they work very well, and when they stop working you are obliged to start replacing magic-boxes because everything in their inner workings is A) sealed off so you can't touch the transistors, resistors, etc., and B) you have to know more about electronics to fix the things than most people have at their command. I have done some work in this area, but I wouldn't feel comfortable trying to instruct you in the intricacies of solid-state electronics even if that did not require more space than can be given the subject here - my ignorance is too great. One thing I can tell you is that all the magnetically-triggered systems should be timed using a "strobe" light; it is impossible to set them accurately by any other means, and the best of these systems, badly timed, is inferior to the conventional battery-and-coil, or magneto. Proper timing is the first requirement with any ignition system.
Finding the exact ignition advance yielding best results with a given engine is, as noted at the beginning of this chapter, a time-consuming chore, but one worth pursuing diligently. Actually, unless you have changed your engine very considerably from the stock specifications you probably will find that maximum performance is obtained at a setting very near that recommended by the engine's manufacturer. Generally speaking, modifications that tend to raise engine output without increasing the crank speed at which maximum power occurs will require that the spark be retarded slightly from the standard setting. Major upward relocations of the power peak usually require a more advanced spark. I would be delighted to provide you with a universally-applicable rule for predicting the exact spark timing for two stroke engines; unfortunately no such rule exists. It is known that about 75-percent of the combustion process should be completed by the time the piston has reached TDC, and that when the average rate of flame-front travel can be found, calculating spark advance from the flame propagation rate and the distance from the spark plug to the remote end of the combustion chamber is a simple arithmetical problem. But confusion arises because flame propagation rates vary so enormously. Both the type and location of the spark plug has an influence in this, as do pre-combustion turbulence and the shape of the combustion chamber. Engine speed also has an effect, and as your modified engine probably departs from the original specifications in all of these particulars, it is essential that you do the careful testing necessary in finding that elusive optimum. Start testing with the ignition timing retarded about 5-degrees from the stock setting, and then advance the spark in two-degree increments until best results are obtained. Dynamometer testing is the best means of verifying results, but you can also use a drag strip, for it has been found that while sheer rider-technique is the determining factor in elapsed time, the motorcycle's speed at the end of the quarter-mile is almost exclusively a function of engine horsepower. Thus, if you find that maximum speed (which may be taken from an accurate tachometer) is reached with a spark advance of, say, 27-degrees BTC, then that timing will be the one providing maximum horsepower. Interestingly, the influence of mixture turbulence - which increases with engine speed-is so strong that the spark-advance optimum does not change much even over a fairly wide speed range. Indeed, from all available evidence, the ability of combustion-chamber turbulence to speed burning is so great that increases in peaking speeds frequently require a slight retarding of ignition, while advancing the spark may well give a small improvement in power at lower-than-peak engine speeds at the expense of maximum power.
People who are relatively inexperienced in the art of tuning racing two-stroke engines seem always to fall into the trap of trying to use a spark plug that is too "cold", and / or one having the wrong nose configuration, which they then compensate with a too-lean mixture. This kind of error probably stems from a basic misunderstanding of spark plug heat-range, and the reason for having more than one type of plug. The reason? As it happens, the temperature of the burning mixture in the combustion chamber is high enough to melt the engine, and it fails to do so only because heat is carried away from the combustion chamber's walls fast enough to prevent them from reaching flame temperature. The spark plug reaches much higher temperatures than the cylinder head itself, because heat moving away from the plug's nose must cross the joint formed, by the threads on the plug and in the plug hole. And,of course, the exposed plug electrodes are separated from the cylinder head's cooling fins by -a very long heat-path, and in the case of the center electrode, by the plug's insulator. Consequently, the electrodes get extremely hot, and that is very much a mixed blessing: Fairly high temperatures are required to keep the plug's electrodes and insulator tip burned clean, to prevent the formation of sooty deposits that can short-circuit the spark. But if the electrodes are allowed to become too hot, they will constitute an independent and uncontrolled ignition source. That is to say, the electrodes may become hot enough to ignite the air/fuel mixture in advance of the spark. When that occurs, combustion chamber temperatures climb even higher which causes auto-ignition still earlier and that yields yet higher temperatures in an ascending spiral that quickly leads to outright engine failure.
So, the correct spark plug for a given engine is one that stays hot enough to burn its electrodes and insulator tip clean, but does not reach temperatures high enough to cause auto-ignition (more commonly termed, "pre-ignition"). Unfortunately, all spark plugs transfer heat from their electrodes to the cylinder head into which they are threaded at a rate fixed by their construction, while engine operating temperatures vary enormously. "Correct" selection of a spark plug is therefore a matter of choosing one having a heat-transfer rate compatible with the application at hand. The process of selection is less exacting today than in the early days of the internal combustion engine, principally because the mica and porcelain used as insulators have been replaced by aluminum oxide ceramics that are stronger and much better conductors of heat. Other improvements in spark plug construction have further widened plug heat-range, hut not to such extent that any plug will perform satisfactorily in any engine. Touring plugs have extended tips, which burn clean but have exposed ground electrodes and center electrodes, and these respond to prolonged full-throttle operation by becoming incandescent; racing plugs have short, shielded electrodes and will tolerate higher combustion chamber temperatures but foul very quickly under starting and idling conditions.
The most common spark plug configuration is the one in which the ground electrode extends over the end of the center electrode. A much better type, in any two-stroke engine application, has the ground electrode reaching in slightly lower to approach the center electrode from its side. The improvement, here, is that the ground electrode is somewhat shorter, and being shorter offers a more direct heat path to the plug body - which means that it is less likely to become white-hot. A refinement of this design has a short bit of platinum wire inserted in the tip of the ground electrode. This wire's diameter is quite small, and it is used in conjunction with a center electrode also reduced in diameter at its tip, an apparently minor difference unless you consider that much less voltage is required to form a spark between two points than between two flat surfaces. An ordinary iron ground electrode cannot be made pointed, because its tip would instantly overheat and melt, but platinum -with a melting point of 1774oC, as compared with 1535oC for iron - is less likely to melt, has a thermal conductivity 18 times greater than iron, and will not oxidize. As a result, the platinum electrode survives its hostile environment very well even when used in small diameters, and I would not hesitate to recommend the platinum-tip plug for most racing applications. At any given heat-range, the platinum plug will provide longer life and less tendency to foul than any conventional type plug. Pure racing plugs, which have deeply recessed insulators and center electrodes, with an extremely short ground electrode bridging straight across from a hole through the side of the plug body, should be used only as a last-resort in two-stroke engines. Racing plugs of the type described are very, very prone to wet fouling, oil fouling and every other kind of fouling possible. They are a necessary evil in hyper-horsepower road-racing engines, but should never be used where extended-nose plugs will serve. Not unless there is some kind of major breakthrough in either spark plug or ignition system design, which always is a possibility. I would recommend that all who are serious about keeping ahead of the pack keep abreast of developments in these areas. All the spark plug manufacturers are working constantly to give us improved foul resistance and reduced spark-voltage requirements, and they are very accommodating to anyone who takes the trouble to write and ask for literature.
"Reading" spark plugs, and the process of selecting correct heat-range, falls much more into the realm of art than science, and it is an art in which one becomes really proficient only after long and sometimes painful experience. But there are a few rules that may be used for guidance by those who have yet to acquire experience - or by the many whose experience has left their ignorance largely undiluted: First, you should know that it is all but impossible to read anything in the appearance of a spark plug unless the engine has been cut clean after having been brought up to operating temperature and given a long burst of wide-open throttle. Very experienced tuners will see the signs they're looking for under the layer of soot, oil and fuel that 'accumulates so quickly at idle, but even they vastly prefer to work with clean-cut plugs. Second. get the right heat-range before you try to rend mixture strength, and my recommendation is that you always use the hottest plug the engine will tolerate. You'll know a plug is too hot when you observe signs of blistering around the insulator nose (which will also be scorched white) and on the electrodes. A too-lean mixture will also give you a whitish insulator, but will not usually produce the burned, pitted appearance of the electrodes that is characteristic of a too-hot spark plug. Also look for signs of melting along the sharp edges at the ground electrode's end – any sharp corner will get hotter than other areas along the electrode, and trouble will first be revealed there. A plug that is too cold simply looks, and is, wet. Plugs of the correct heat-range get hot enough to burn away oil, and soot, and will have only dry, brown to tan deposits on their insulators after a hard run. As noted before, the correct mixture strength will be wry slightly leaner than that which is just lean enough to keep the engine from four-stroking. How much leaner? Not very much, and until you have gained considerable experience with a particular engine you should not reduce strength below the jetting that provides clean running. To get a bit closer to the optimum, I watch the faint, almost invisible ring of soot that forms around the electrode on the insulator's nose, and the light dusting of soot over the exposed end of the plug body. There is a point at which I see "just enough" soot, and if there is more or less than that - I interpret the signs to mean a mixture that is too rich or too lean, respectively. And I couldn't begin to tell you how much soot is "just enough”; that would be like trying to explain a taste, or sound, or smell. With experience, you learn to recognize what it is you're looking for, and there is no substitute for that experience. You will also learn - if you know where to look - that the faint light spot on the electrodes at the sides of the spark gap are an important clue to ignition system performance. When you have a spot of about the same diameter as the ground electrode's end showing on the center electrode, or vice versa,then you may be sure the magneto is doing its job. When that spot begins to fade, or become ragged around the edges and shrinking in diameter, the ignition system isn't performing as it should. Finally, with experience you'll learn to give your very close attention to all aspects of the mundane task of selecting jets and plugs, and spark timing, because in these things you ultimately succeed or fail as a tuner; all the rest is mere mathematics, surgery and wrench-twirling.