Contrary to what you may have read—and speaking exclusively of Kettering systems, by far the most common—primary resistance is the least important aspect of an ignition coil in regard to its compatibility with an ignition system. (It’s compatibility with the charging system is however another matter.) Despite this, we often think in terms of 2-ohm, 3-ohm and 5-ohm primaries, and everywhere is found advice on testing coils by measuring their primary resistance. The plain truth is, all of these coils can be used in all Kettering (traditional and transistorized) type bikes with no ill consequence. Mix 'em up, it doesn't matter. You’ll never know the difference. And as ar as testing, no ignition coil is properly tested by measuring its primary resistance, a fact career mechanics learn pretty quickly.

Check for cracks in the ignition coils. Kettering coils overheat. Make all the connections clean and tight, including the wire harness connectors, keyswitch and engine kill switch, all of which are exposed to the elements and which directly affect coil voltage. Pre-1979 Honda ignition coils have spark plug wires that are molded in place. Honda’s plug wires are multi-strand stainless steel, so barring physical damage they are lifetime parts. On the other hand, all U.S.-spec automotive and most aftermarket powersports plug wires (which are made by auto parts manufacturers) are like Honda’s plug caps made of carbon and therefore degrade quickly like the caps and are very difficult to make good electrical connections with—the reasons in fact car mechanics replaced them frequently. If you are buying aftermarket ignition coils with removable plug wires order them with stranded copper metal plug wires. Unscrew the spark plug caps occasionally and resistance-test them. Replace pre-1982 caps when they “ohm” above 5K or later caps that exceed 10-15K.

Fitting 1990s model ignition coils to 70s Hondas is pretty popular. As long as the replacement is Kettering-to-Kettering, and the primary resistances are therefore between 2 and 5 ohms, there is no problem. However, putting CDI-intended coils or any coils likewise having less resistance than 2.2 ohms is liable to result in some problems with vintage bikes’ marginal charging systems. And putting “stick coils” on vintage bikes, which seems to be an Internet trend, is a bad idea for another reason. Stick coils—technically coil-over-plug coils—are actually lower-powered coils than the original, not higher-powered as many assume. Chalk up one more fallacy to the knee-jerk character of the Internet community.

Most on-road powersports vehicles come with resistive (“suppressive”) spark plug wire, or resistive spark plug caps, or resistor spark plugs. Honda roadbikes produced before 1980 have only one of the three, the resistive caps. In 1980 Honda began adding resistor plugs to comply with Canada’s then-new radio freqency interference (RFI) requlations. Today’s digital communications are said to not be subject to this kind of interference, so the radio wave controlling aspect of resistive ignition compoments on pre-1982 Honda road models may now be irrelevant. And since they are carbon-based and therefore deteriorate over time, defeating resistive plug caps offers an advantage. But on Hondas made after 1982, unwanted RFI can interfere with some of the bike’s onboard systems, namely the LCD instrumentation, and on many models made after 2000, their computers, servos and gyros are at risk. So for all those later bikes the resistive plugs and plug caps should stay. Any bike onto which you have clamped a GPS or audio device or smartphone could be affected also. In these cases as well, at the very least keep the resistive caps.

High-voltage ignition coils
The aftermarket has convinced us that fitting ignition coils having a higher voltage potential than stock will necessarily increase engine performance. This is a myth. If your completely stock bike is struggling with engine performance issues related to fueling shortcomings, then yes, zapping it with a little stronger ignition can help. It can make up for carburetors that for whatever reason are not working their best. Yes, a stronger spark can improve carburetion, by compensating for the resulting impaired combusion. But of course the carbs in that case should be dealt with instead. Realize that an ignition coil never delivers its maximum, but only that voltage required at the spark plug at any given moment. This varies constantly. At idle it’s maybe 5,000 volts. In gear, with loads and temps and other conditions varying, up to and exceeding 10,000-plus volts. And more when conditions demand. But it’s only when the engine’s poor tune consistently demands more than usual from the coil that the stock coil may become inadequate and a different coil might possibly be an advantage. But again, fitting a higher-voltage coil in that case is the wrong way to fix things. It’s like giving cough medicine to someone with bronchitus. Give attention to the real problem.

Stick coils
There is a trend of fitting modern stick coils to vintage motorcycles. But this comes from a significant misunderstanding. Far from being high performance ignition coils, stick coils are virtually the lowest-voltage coils ever in the long history of the internal combustion engine, and certainly the weakest coils found on machines in the last several decades. The big reason for stick coils on modern engines is individual cylinder control. But what made these very small, low-voltage coils work despite being low-energy is the fuel injection found on individual cylinder controlled engines. Fuel injection is so much less a compromise fueling-wise than is carburetion that very liitle spark energy is needed. This is a prime example of the symbiotic nature of fueling and ignition.