Virtually all 60s and early 70s Honda roadbikes use the traditional Kettering ignition system. Invented in 1908, this ignition is rock-solid and stone simple. The one-shot transformer that is the ignition coil is simply powered up then turned off, and at that point we get spark at the spark plug. Therefore, this is called a “collapsing field” system.

Because the coil carries current whenever the keyswitch and engine kill switch are on, it stays on for long periods and tends therefore to run hot. This is its main failing. The coil is manufactured by wrapping thin insulated copper wire around an iron bar. This is the coil’s secondary winding, the one connected to the spark plug. On top of this is wound another layer of thicker insulated wire which is connected to the battery. This is called the primary winding, and it is controlled by the ignition contact points. While the points are closed, the ignition coil’s primary winding is voltaged—current is flowing through the winding and a magnetic field surrounds it, concentrated in its place by the iron bar. Then when the points open, this current flow is broken and the magnetic field disappears—we say it collapses. This collapse falls inward toward the coil’s inner “secondary” winding, which magnetically induces it to produce the voltage that is applied at the spark plug. The speed of the collapse significantly determines the strength of the secondary winding’s induction. Points that are dirty or exhibit electrical arcing slow this collapse, reducing the coil’s performance. The condensor found in the Kettering system ensures that even clean points won’t affect the coil’s output by arcing away some of the primary winding’s energy. That’s the condensor’s purpose. Reduced points wear is merely a bonus.

Despite its simplicity, there are a number of possible failure points in Kettering ignition. Misadjustment is probably the biggest fault, and this is increasing in our day when few of the owners of these bikes are able to appreciate the importance of servicing the ignition system. Some are simply too young to have any context with which to respect ignition’s role in engine performance. Honda’s points are very long-lasting, requiring only occasional dressing and retiming. A 20,000-mile life is well within reach. The coil is easily tested for failure by simply powering it up and intermittently grounding it. This can even be done with the coil left on the bike. When a more conclusive test is needed, installing a test spark plug whose gap is increased to 0.125” will stress the coil to its maximum. But beware—even coils that pass these tests can be “tired” and have dimished performance. The ignition coil over 40 to 50 years old will gradually decrease in strength without actually going bad. The proof is when replacing the coil increases throttle response, idle smoothness, and overall engine performance. It happens all the time. A resistance test of an ignition coil is a waste of time. Wire windings are least conclusively tested statically. Test the coil dynamically.

“High performance” ignition coils are those whose windings are wound to different ratios than those in the stock coil. The most common example is the coil in which the primary winding has been reduced in the number of its turns, producing not only a taller winding “gearing” that potentially increases induced voltage, but also less resistance and thus greater current flow, resulting in a heightened magnetism. This coil is capable of higher spark plug voltage output. Capable, mind you, because the environment the spark plug “sees” ultimately determines how much of the coil’s available voltage is exercised at any given moment—not its voltage rating. You may have heard of the old two-stroke dirt bike rider’s trick of cleaning a dirty spark plug by loosening the plug wire and holding it a distance from the plug. This dramatically increases the load on the system, resulting in a very high voltage being sent to the plug, in the hope of blasting off some of the carbon buildup. This principle is important to remember because it desmonstrates that the load on the ignition system—specifically at the spark plug—determines plug voltage. Low load, low voltage. Higher load, higher voltage. As a result, at idle the engine is not being stressed, combustion is lazy, and very little voltage is needed. About 5,000 volts gets the job done. In gear and on the road, the plug sees more combustion activity and more stress, and the voltage demand increases, to perhaps 10,000 volts. In extreme conditions: uphill, bucking a headwind, high gear, etc., the coil will then supply the most it ever will in normal use—possibly 15,000 volts—but still not as much as it is rated for. There is always a reserve, the rating of the coil is always higher than is ever needed. What then is the benefit of a higher voltage, high performance ignition coil, if the system adjusts its voltage as needed and practically never is its max potential used? An excellent question and one that will start you thinking properly about ignition coils. The high performance aftermarket ignition coil, having a higher potential maximum voltage, though as with the stock coil that voltage is never required, results in the voltage needed at any given time getting to the plug more instantaneously, more quickly, which can potentially improve combustion. Many find this hard to grasp. But time in this case is measured in thousandths of a second. All the same, it counts, just as the speed of the coil’s primary collapse counts.

I’ve said this elsewhere, but the real outcome of better ignition is better carburetion. Yes. Ignition and carburetion overlap tremendously in their dependency on each other. Poor carburetion makes higher demands on the ignition, thus a badly maintained intake system can to a degree be overcome by stronger ignition. The converse is also true. A weak ignition will get the job done properly only if fueling is tip-top, therefore a richer than usual mixture can often overcome ignition problems.

Though the typical low primary resistance type higher-voltage ignition coil can be an advantage, there are trade-offs. Its lower resistance increases current flow through the points, potentially shortening their life. In practice however Honda’s superior quality points make this of little concern. A more important consideration is this increased current in the coil adds to the bike’s electrical load. On many vintage Hondas, whose electrical systems have only 100 to 200 watts of power, this can be a burden the motorcycle can’t deal with and low battery charge may result. This is of course even more a concern on bikes whose headlights will be on while riding, and the most concern of all on machines whose electrical systems need service due to bad wiring and connectors, Wal-Mart batteries, failing rectifiers, Chinese charging system parts, improper modifications, etc. When it comes to vintage Hondas, electrical modifications such as a higher-powered headlight or a higher voltage ignition coil must be undertaken only on a bike in perfect electrical condition, and even then with some studied knowledge and forethought.

Part 2