Smog and the transistor
The Honda CBX ignition owes its origin to Charles Kettering, the founder of Delco and the first to patent the breaker point ignition system.1 Kettering’s 1907 invention is a marvel of simplicity, and its fundamental design served motoring well into the 1980s, succumbing finally only to the all-powerful EPA.2 As a testament to Kettering's remarkable ingenuity, when the points were superseded, two new parts were needed to replace them. The breaker points’ timing duty was given to the new electronic system's pulsers, and the switching was taken over by high-speed solid-state switches called transistors. Together with the battery, ignition coils and spark plugs, these pulsers and transistor boxes ("spark units") make up the bulk of the CBX’s ignition system.
Here’s how these parts work together to make the spark plug "fire." The key is turned, and battery voltage is fed into the ignition coil, saturating it with a magnetic field. Independent from this, the electric starter is activated and the engine turns, causing a pulser driven by the crankshaft to generate a tiny (1v) signal that is fed into the spark unit. The spark unit then switches off the power to the ignition coil, which then collapses its magnetic field, inducing a high voltage on the output side of the coil, which jumps across the spark plug’s gap. Though it sounds slow, it all happens in just thousandths of a second.3
The spark event up close
The description above points out something important -- the ignition coil is "hot" whenever the key is on. This is important because leaving your keyswitch on without the engine running can overheat the system’s spark units. In fact, overload such as this is the number one cause of spark unit failure. But there is more here. Let’s zoom in close and check a few things out. Notice that in our system the coil’s voltage builds (2) until it reaches the level needed to make the air in the plug’s gap conductive (3). Engineers call this "ionization." Once the gap is ionized, the voltage necessary to continue current flow is less than that required to initially get across the gap, so the voltage drops to the minimum necessary to keep flowing across the gap. The energy dissipates (the voltage drops) as it flows (4) because it is limited (by the level of coil saturation), and keeps falling until it is no longer enough to continue arcing across the gap. At this point the spark stops. With the load sudddenly removed from the coil, the voltage surges upward momentarily and the plug gap comes close to re-ionizing. But, because the coil now lacks the voltage required to do so, the energy is quickly spent. The coil has made its last "hurrah" and the voltage at the plug falls off without re-firing the plug (5).
Voltage build-up and resistor plugs
This monkey-motion illustrates a number of important things. First, notice that the coil’s high voltage does not arrive at the plug instantaneously. This is important because it explains much about ignition systems in general and specifically why coils rarely use all their voltage. It can be seen from this that the CBX’s 28,000-volt ignition system does not fire its spark plug at 28,000 volts. Instead, the plug fires at the lowest voltage needed to bridge its gap at a given moment. Note: at a given moment. Voltage depends on load and in a motorcycle engine this means many things, but it's easiest to think rpm and gear position. The plug fires at around 5,000 volts at idle, and about double that during acceleration. Only under the most extreme conditions does an ignition system use more than two-thirds its capacity. However, the coil’s high capacity is not wasted. The extra voltage is needed because some voltage bleeds off the plug through carbon fouling during the ionization process. Increased voltage producing ability supplies the push that ensures rapid spark voltage rise and a robust spark despite this inevitable loss. It also provides a safety margin for those severe conditions. In fact, there are three ways high output coils improve engine performance. First, by providing more surplus voltage, the spark event has more push behind it, making the spark plug less susceptible to fouling-induced misfires. Second, with more voltage to spare, the spark plug gap may be increased, whose larger resulting flame kernal more vigorously starts combustion, resulting in a real-world increase in engine torque. Third, higher available voltage results in increased coil saturation, and ultimately a longer-duration spark that more easily ignites less-than-perfect air/fuel mixtures. Another thing that is visible in the spark event is the rapid oscillation in the firing arc. This sawtooth voltage is electrically "messy" and actually creates radio waves that interfere with radio reception and with sensitive electronics of various kinds. Resistor spark plugs damp this oscillation, lowering the frequency of the radio waves nearer to that of ambient levels.
What then is CDI?
Many people mistakenly call the CBX’s ignition CDI, and even some early factory literature made the same mistake. CDI (for Capacitor Discharge Ignition) was developed to fight plug fouling on early two-stroke snowmobiles. CDI is different from transistorized ignition in many significant ways, but especially one relevant to this discussion. CDI has one less step in its firing process, eliminating the need for the coil to power down before the spark plug fires. As a result, there is much less time in which plug deposits can bleed off part of the firing voltage, making the system largely impervious to fouled plugs. Unfortunately, in its original form, CDI also had a very short spark duration. This made it unsuitable for street bikes whose lean mixtures need long-duration sparks to burn correctly. By the early 1990s however, OEMs started using multiple-spark CDI to lengthen the spark event, thus creating the best overall ignition system available during that period. As you can imagine, due to this inovation and many others besides, ignition systems are no longer simple. Today’s powersports ignitions are integrated into the vehicle’s fuel, charging, and electric starting systems. Ignition systems have controlled carburetor jets, advanced or retarded by the vehicle’s computer to increase alternator rpm, and a lot more. Today they also fire different cylinders at different degrees of advance, and fire in dizzying patterns to avoid unsafe high engine rpm.
Troubleshooting pulsers and spark units
Most pulsers produce about 1 volt, and that so very briefly. A multimeter has a hard time with this, so a special tool called a peak voltage adapter is connected to it to pick this transient voltage up easily. It freeze-frames it. If a peak volt adapter is not available, test the pulser with an analog multimeter set to the picoamp (uA) scale. A small but rhythmic needle twitch as the engine turns over indicates a working pulser.4 As a backup test, I like to substitute a 9v battery for the pulser. If the pulser doesn't make the system spark and the 9v battery does, the pulser is at fault.5 As for the spark unit, sometimes it will fail when the bike is started for the first time after sitting. If the battery is very low, its voltage may fall below 9.5 volts during starting. If this happens, the transistors in the spark unit can fail. They overheat because they never turn off and do not therefore dump the current flowing through the ignition coil. Test the spark unit by testing the parts on either side of it: the pulser and the ignition coil.
1 Kettering (ever hear of the Sloan Kettering Cancer Center?) also perfected the first truly efficient electric starter and was the leader in developing tetra-ethel lead for use as a gasoline improver in the 1920s.
2 And that only due to a little-known clause in the 1978 Clean Air Act which required all subsequent ignition systems to be essentially maintenance-free by virtue of the requirement of not affecting emissions for five years.
3 Actually, from the spark plug's perspective, this is slow.
4 This was at one time a factory Yamaha authorized technique.
5 This was at one time a factory Kawasaki authorized technique.
Go to Part 2