Issue link: https://read.dmtmag.com/i/714189
76 nSeptember 2016n www.thunderpress.net THUNDER PRESS Here's the point OK… my friend Sportster Paul will understand my motivation, but suffi ce to say for you, gentle reader, this time out the story is intended to be more cathartic than anything else. Even though we'll get into igni- tions, it all starts out with a support- ing player in points ignitions, the auto advance unit. I've always had a real attitude problem where those damn things are concerned, since eight or nine times out of 10, they are the real problem with points ignitions. For those not versed in antiquities, an advance unit is comprised of a metal plate with strategically-placed posts and holes, a couple of small coil springs attached to pivoting counter- weights, cradling a so-called points cam. The theory at work in this design is the springs hold the weights in towards the center of the mechanism to effectively retard the timing for easy starting (usually by kickstarter) and once running above about 2500 rpm the weights are fl ung out by centrifugal force to advance the tim- ing for higher rpm road work. Two issues; well, make it three. First, when new they are pretty trouble-free, so forgotten by owner and rider until… second, they wear out… then third, in that state, cause hard starting, variable timing, backfi ring, misfi res and other such issues to the point that the points ignition becomes more trouble than it's worth. Especially for those who aren't into care and maintenance and would rather just ride. One other thing—can't replace 'em if they're obsolete, or shouldn't… if of inferior quality! One other "other" thing! Two- strokes, bless their stinky, smoky hearts, don't use advance units. These old strokers have ignitions that are plenty good enough for high perfor- mance and do not require anything like the intensive maintenance that's come to be mythologically associated with points. Sure, rubbing blocks wear and the spring band on points can "bounce" off the points cam at fi ve-digit red-lines… but it amounts to nothing compared to the agro of auto advance in four strokes! This is the position—the only posi- tion—I'll take against these old-school mechanical ignitions. Because aside from that, they have advantages of their own (not least, they will fi re with a near-fl at battery and a good kick- starter). Fact is, electronic and, more recently, digital ignitions have little or no performance advantage over prop- erly maintained points, beyond being "maintenance free." But… To point out When points close, current fl ows through the coil's primary winding, magnetizes the coil core, and stores magnetic energy. When points open current fl ow is interrupted, causing the magnetic fi eld to collapse. This rapid cut-off induces a large voltage in the coil secondary windings. The faster the rate, the higher the voltage. An inductor (a.k.a. ignition coil) resists this change in current. As current col- lapses, making that nice, high voltage to fi re the spark plugs, the other end is also generating high voltage trying to suck electrons across the open points. If the voltage gets high enough, cur- rent fl ows across an arc, and will stop collapsing and no high voltage will be generated and points simply fry! This is where the condenser (which every other industry in the world calls a capacitor) plays its role. When the points are closed, the condenser is also discharged. As the points open and the coil tries to suck electrons, the condenser acts as a reservoir, pro- viding an electrical cushion, until the points get far enough apart to prevent formation of an arc. It's a balancing act… and essentially a switch. A pointless exercise Almost all ignition systems work like this, so what if you could replace mechanical contacts with a solid-state device that is not subject to wear? Solid-state device? Okay, okay—a transistor or a microchip. You turn it on, the resistance drops, and current fl ows (just like the points closing). When it's turned off, the resistance goes up (way, way up), and virtually no current fl ows. Since we no longer need to slow down the voltage rise to allow time for the points to get out of the way, the coil current can be switched off much faster. This results in a faster cut-off and is a higher sec- ondary voltage. Additionally, since this thing is a solid chunk of silicon, there is no opportunity for creating an arc. Some early electronic ignitions (most notably Japanese in the early 70's) were actually hybrids that used points to control the timing and a transistor to switch the coil current. Although the points lasted much lon- ger, the system was far from mainte- nance free.They were prone to dwell shift due to rubbing block wear, con- tact corrosion near marine environ- ments, insuffi cient current to prevent oxidation of the contact, etc. So, the next step was to create some form of non-contact sensor to generate the timing information. The big three are: magnetic, optical, and Hall effect triggering. Trigger happy Magnetic triggering has been used by virtually every manufacturer since the mid-70's and is still widely used today. Typically, a bar of steel is wrapped with several hundred turns of fi ne wire on one end. A small magnet is attached to the other end, and this assembly is mounted in the distributor facing the distributor shaft. Where the point cam would normally be, a small-toothed "reluctor" wheel is attached. As the teeth approach the coil, the fl ux from the magnet is pulled in close to the bar. As the teeth move away, the fl ux springs back out- ward, inducing a voltage in the pickup coil, used to drive a high voltage/ high current transistor that switches the coil current. Used mostly in cars, magnetic triggering has limited abil- ity to sense teeth that are very close together, which is necessary to gain the positional accuracy required by modern engine management systems. Optical triggering consists of an infrared LED (light emitting diode) facing a phototransistor separated by a small gap. Through this gap a slot- ted wheel passes which alternately blocks and unblocks the light, gen- erating position information. Since light will pass through a very nar- row slot, a high degree of positional accuracy can be obtained. It was the only viable alternative to magnetic back in the 1970's when most of the aftermarket ignition companies were founded. It was attractive chiefl y because a simple trigger wheel could be fabricated out of plastic or other household materials and the output required minimal signal conditioning, unlike magnetic. A so-called Hall effect trigger consists of a wafer of silicon through which a current is passed. When a magnet is placed in proximity to the wafer, the current tends to bunch up on one side of the silicon. This con- centration is amplifi ed and detected, indicating the presence or absence of a magnetic fi eld. The advantages are, since it is an integrated circuit, it can be made very small with a number of features at minimal cost. It exceeds all current automotive temperature specs, and its accuracy is unaffected even This mechanical marvel is the root of most evil in points ignitions… or any ignition that uses one. If (and it's a big "if") they are properly maintained, lubricated, and well made (properly sprung and weighted in the fi rst place) they are merely adequate for the task of retarding and advancing ignition timing.