Resistances There seems to be a lot of misunderstanding about the resistive parts of Honda ignition systems. Let’s bring a little clarity to bear.

We need to slaughter the most massive sacred cow first: there is no Kettering-equipped Honda streetbike on which a resistive ignition component aids ignition performance. Talk all you want about the resistor heightening spark energy or extending the spark event. That’s completely propeller-head and no one has ever proven it. To the contrary, there is the experience of many industry professionals over a half century that seriously questions the idea. Consider for example that a resistive plug cap that over time only moderately increases resistance negatively affects engine performance. That’s why they must be replaced periodically. Another hoary myth is that resistive ignition components exist for only one reason, whether it’s the aforementioned performance theory or some other. The truth is there are a handful of very different reasons for resistive ignition parts.

Historically, resistive ignition parts began as a concession to radio freqency interference (RFI). Honda started with resistor plug caps and later added resistor spark plugs. It is likely that Honda put resistor plug caps in the beginning on their bikes in a sort of concession to how messy stray electrical waves were viewed by their engineers. Such was their original purpose, to damp RFI, though that early no outside authority had asked them to do so. It was simply a good thing to do, as it protected your neighbor’s viewing of the late late show on TV. In 1980 however, according to a Honda service bulletin, new RFI regulations in Canada (proposed to protect emergency services radio tyransmissions), a significant part of Honda’s sales market, prompted Honda to add resistor spark plugs to their already existing resistor plug caps, and these resistor plugs appeared on US models as well (to streamline production). Much later, after the turn of the millennium, the Big Four would make further changes in their ignitions, again to reduce RFI in response to some Asian market RFI requirements. So RFI has long played a huge role and is the original and the major reason for Honda’s resistive ignition components.

Another reason for ignition system suppression is electrical feedback in the wiring harness that can affect other components, whether onboard computers, LCD instrumentation, or even the ignition system electronics themselves. There are many modern powersports vehicles for example whose engines will run badly or not even start if non-resistor spark plugs are installed, and at least one of the Big Four has made changes to their ignition systems solely to reduce the effect they were having on their vehicles’ fuel injection system throttle position sensors, which were failing prematurely due to the electrical “noise” created by the ignition system. And the noise is definitely there, as an ignition oscilliscope will demonstrate, and as modern peak volt measurement troubleshooting techniques also confirm: a 12-volt powered Kettering ignition can backfeed as much as 200 volts into the wiring harness.

Vintage Honda ignitions can include three types of resistive ignition components: resistive spark plug wires, resistive spark plug caps, or resistive spark plugs. Hondas have had up to two but in my experience never all three—I have never seen suppressive spark plug wire as original equipment on a Honda. Many advocate removing the resistive elements from Honda’s Kettering ignition. While every rider would do well to get after more fundamental tuning related maintenance before thinking about doing this kind of modification, there are potential benefits to getting rid of ignition secondary resistive components on vintage Hondas. For one, removing the resistor caps eliminates them as a future failure point. They grow more resistive with age, and definitely affect ignition performance as a consequence. In addition, if the caps are replaced with old-school, 1940s-style soldered-on terminals, that’s one more maintenance point removed, as Honda’s screw-on caps tend to loosen and corrode and thus require regular attention. Another benefit is the freeing up of slightly more potential spark plug voltage, which can be utilized by slightly increasing the spark plug gap. This can result in improved idle and acceleration, especially if the bike is plagued with carburetion problems. Ignition and carburetion overlap, remember?

One mistake often made relative to resistive ignition components on vintage Hondas is the purchasing of aftermarket ignition coils that typically are shipped with automotive suppressive spark plug wires. Using those plug wires adds resistance where it was never needed, and worse, introduces non-metallic crimp points whose conductivity starts out bad and can only worsen from then on. There really is no reason to put carbon-based plug wires on a vintage Honda. The stock wires are stainless steel and never go bad. The worst that can happen to them is the rubber insulation hardens up after fifty years and becomes prone to cracking.

While on the subject of resistance and ignition systems, it is important to remember two things about Kettering ignition coil primary resistance. One, this is not how professional mechanics evaluate ignition coils. Measuring the resistance of winding-based components for the purpose of troubleshooting is a waste of time. Or more accurately, is a waste of time whenever the reading is good (see the 60/40 Rule). Second, although some folks want to make much of an ignition coil’s resistance and its subsequent suitability for an ignition system, within the Kettering system, it just does not matter. You can’t hurt the points by your choice of a coil, you can’t hurt the transistors (in later systems) by that choice either, and nothing else in the system is subject to issues. Assuming of course that you stay within the normal 2.2 to 5 ohm range of Kettering coils. And even then, going below 2.2 ohms will not hurt the ignition system, but it probably will have an effect on smaller vintage Honda charging systems that will be overcome by the extra current load.