In The CBX Ignition, Parts 1 and 2, we looked in detail at the pulsers and spark units, including how to test them. In this, the final installment, we’ll take a close look at the ignition coils.
Hummm, Zaaap! – Hummm, Zaaap!
Most people know that an ignition coil is actually a transformer. It steps up the battery’s 12 volts to a potential of thousands of volts. We’ve learned so far in The CBX Igniton that ignition coils do their work in short bursts initiated by the pulsers and prompted by the spark units. Like a transformer, the ignition coil is made of two windings. As explained in Parts 1 and 2, the coil is saturated first, then turned off, resulting in the collapsing of the magnetic field in one of the coil’s windings leading to the rapid build up of voltage in the other. Other than the substitution of pulsers and transistors for the age-old points, there is nothing here that is different from the old Kettering ignition that powered that old Honda 350 twin you had back in high school. It’s a pretty ancient design. I know, I keep emphasizing this. But it’s useful to keep in mind because it prepares us to look for some of the same problems as are found in the Kettering system, and it helps us know how to tackle them.
Two Plugs Per Coil
If there is anything that is different from that old 350 however, it is how the CBX’s ignition coils are wired. The CBX has just three ignition coils for its six cylinders. How it do dat? Well, just as with many late model cars, each coil handles two cylinders, with the spark plugs in alternate cylinders in each pair firing on the exhaust stroke instead of on the compression stroke. This is set up by three things – the engine’s 120 degree crankshaft, the cams’ timing, and the arrangement of the ignition system’s pulsers. The result is what is called the "wasted spark" system, a system that is still in use today on many motorcycles. The really odd thing about all this is that the two plugs attached to each coil fire in series, not in parallel. That is, they fire in line with each other. In other words, one plug always fires from the center electrode to the outer electrode, and the other, after its current speeds through the cylinder head casting, the opposite way, from the outer electrode to the center electrode. The engine doesn’t give a gnat’s behind which way the spark jumps, forward or backward. However, the spark plugs do. The plug firing from the outer electrode to the center electrode tends to foul easier, because due to its blunter firing edge, it needs more voltage to fire than the other plug does. Three of the six plugs are in a sense handicapped. This is in fact the basis for the design of both the u-gap and split-gap spark plugs. Such plugs have ground electrodes designed with machined sharp edges. The result is that there is no difference in the voltage required to fire the plug in either direction. That’s the theory anyway, and Honda must of thought it worth a try because a certain percentage of Hondas made at the time of the u-gap plug’s introduction in 1977 were supplied with them as orginal equipment. By the way, you can verfify which plugs fire which way on your bike, very easily. Wait until they have at least 3,000 miles on them, and you will see two different wear patterns on the six plugs. The three that have been firing "forward" will have rounded center electrodes. The three "backward" firing plugs on the other hand will have little wear cavities on their ground electrodes. Or, if you’d rather not wait, you can lay a sharpened pencil near an exposed firing plug, and the flash at the graphite tip will indicate the direction of the spark. Just don’t hang onto the metal at the eraser end!
High Performance Coils
High performance ignition coils for the CBX are much in evidence these days, even more than when these bikes were new. High performance coils are very simply just coils whose windings are differently proportioned than the stock coils. That is, the winding’s turn ratios are different. In the best examples, the secondary winding is increased and the primary winding the same as stock. The difference in turn ratio results in a higher potential voltage output. "Potential" because, remember, no coil fires at its voltage rating, but rather at the precise voltage that will bridge the spark plug gap at any given moment. What’s the point of high voltage coils, then? We saw in Part 2 that, aside from easier starts and reduced plug fouling, due to the coil’s faster output voltage rise. the extra voltage allows you to run slightly larger plug gaps and get a mild increase in low rpm power.
Beware of Low Resistance
But there are also high performance coils where the secondary turns are left same as stock while the primary turns are much less than stock, resulting in a similar increase in turn ratio, and a more compact, more economically produced part. The change in the primary winding’s turns is detectable as a difference in primary resistance, measured of course in ohms. However, these coils are designed specifically for ignition systems that either, 1 – are points-based and therefore do not have transistors, or 2 – have transistors but have them wired to the coils in a rising field setup rather than a collapsing field one. Aftermarket CDI, for example. Remember, the CBX ignition is the same as the Kettering in that the coil is "hot" 90 percent of the time and only turned off to create a spark. The problem is, Kettering-based transistorized ignition systems won’t tolerate having their primary winding resistance decreased. The increased current flow will cause their nearly-always-on transistors to overheat and fail. As mentioned in Part 2, be sure you get an aftermarket coil whose primary resistance isn’t so low that the resulting high current flow burns out your spark units. Most aftermarket parts suppliers give you the option.
Maintenance and Testing
The ignition system is more than just these three major parts however. There is a lot of wiring and connectors, don’t forget, and other things. Poke around a bit before going after the major parts. Probably the most high-maintenance part of the CBX ignition system is the plug wires, which we haven’t mentioend yet. However, the reason for this need for maintenance is not what many think, that the wires are like automotive plug wires and need replacing every so often. Hondas never need their spark plug wires replaced. Unlike car wires, they are permanent, stainless steel wire that never breaks down. However, like car wires, their terminals can corrode and get loose. So check them. Remove the plug caps and resistance test them. They should be around 5K ohms. Also make sure the caps are screwed onto the wires tightly, and the wires to the coils. And, look for signs of arcing from the wires to the frame. After looking for the obvious things, then go on to component testing. However, don’t get in a dither about how to tell which of the three major parts in your CBX’s ignition system is not working correctly. It’s easy. In Part 1, we looked at testing the ignition pulser. All you need is an analog multimeter. Part 2 showed us that the best way to test the spark unit is to test around it, and thereby either eliminate or indict it, as the case may be. If you have no spark, and after visually checking the system and then testing the pulser and ignition coil they test good, the spark unit is the problem. The ignition coil is perhaps the easiest of the three to test. Remember, all the ignition coil does is turn on with the keyswitch, then turn off at a pulse signal, and the spark plug fires. To test the coil, first check for battery voltage at the black/white wire, with the keyswitch turned on. If that’s good, then disconnect the colored wires from each of the three coils (blue, yellow, and pink) and simply jump a wire from the battery’s negative terminal to the exposed terminal on the coil being tested. Disconnect and reconnect the jumper at the coil intermittently. Each time the jumper is touched to the coil and then removed, the spark plug should spark. This test simulates what the spark unit does, by grounding the negative side of the ignition coil. Pretty simple, eh? Forget the official manual’s coil resistance test – it’s not conclusive enough. And never mind the vibrating coil tester method, as that is only useful if you’re comparing coils one to the other.
