In The CBX Ignition, Part 1, we focused mostly on the pulsers. In this installment, Part 2, we consider this ignition system’s spark units

Kettering revised
The CBX TPI ignition system is nothing more than the old Kettering points based system revised and made pointless. Hence the name. The interesting part is that TPI needs two parts to do the job that just one set of points did before. TPI’s pulsers (variously called triggers or signal generators) took over the timing chore, while its transistors (spark units or igniters) assumed the switching duty. Nothing else was changed. In fact, all of the parts in both a points system and a TPI system can be completely interchanged. Restorers of old points ignition motorcycles often use ignition coils from later TPI bikes with no ill effects. Moreover, the two systems actually work the same. That is, both battery/point and TPI systems work on a collapsing field principle. They're both Kettering. In the points system, the ignition coil is energized through closed contact points, resulting in a magnetic field inside. When the points open, the coil’s current is interrupted, and the field collapses. The collapse is so sudden that a second ("mutual") induction occurs at the coil, in its larger winding, which produces the spark voltage. TPI works exactly the same way. The ignition coil is energized through a conducting transistor, the pulser sends a timed pulse to the transistor which in turn disconnects the ignition coil, and spark voltage is generated by the resulting collapsing field.

Standard performance
TPI is nothing more than a points system made electronic. There is no extra spark energy. There is no more resistance to plug fouling. No added potential coil voltage. And no reduction in heat cycles (and therefore no reduction in heat related failures). In fact, assuming a carefully adjusted point system, there isn’t even an improvement in timing accuracy. TPI was emissions-mandated not due to increased accuracy, but due to the extended maintenance intervals that emissions warranties required. However, this is not to say that TPI is bad. It’s not. American Honda used the stock TPI on its first-year 1025cc Superbike effort in the early 1980s. However, it doesn’t even approach the technology that is available today, and being a modified Kettering system, it actually has all of Kettering's same drawbacks. Two are serious enough to warrant mention here, and they’re both from the same cause.

First, there is a susceptibility to plug fouling. Because the system works on collapse, the time from pulse to fire is kind of long, as ignition systems go. Where CDI might get the whole job done in just a few milliseconds, TPI needs 20 to 40. Plenty of time for accumulating spark voltage to bleed off before delivering the high powered punch required for combustion. Second, again because of the way in which energy is used in this system, Kettering ignition is a voltage-hog. This means its efficiency is easily upset by small amounts of corrosion in the wiring and connectors.

The spark unit
TPI’s spark units have certain failure modes. A common one is when an attempt is made to start the bike after it has sat awhile and the battery is low. During starting, battery voltage normally dips when the starter is used but with less to start with, the battery’s voltage drops too low for the spark unit to operate, and the engine will not start due to loss of spark. Meantime, the transistor is overheating, and will soon permanently fail. A diagnostic tip: When the ignition system is acting up, but it is mostly when the engine is warm, don’t suspect the spark units. The pulsers are likely the problem. When the ignition performs badly whether hot or cold, then the spark units become more suspect.

Testing
The easiest way to test a spark unit on a later model CBX is to simply substitute one of the other two on the bike. Of course, that only helps if the spark is visibly poor or non-existent, and doesn’t help if you are trying to judge the performance of the engine as a whole. The best test for a spark unit is to test either side of it. Simply test the pulser and the ignition coil. All that is left is the spark unit. Part 1 described how to test the pulser, and we’ll look at how to test the ignition coil in Part 3.

One more thing. It's been observed that fifty year old spark units switch slower than they once did. This results in less plug voltage because the primary collapse is not as abrupt. The rider may or may not notice. But when a new spark unit is installed, the difference is obvious.

Go to Part 3