When Yamaha fielded its two-stroke 250cc TD2 production roadracer in 1969, the level of over-the-counter racers was raised to that of real Grand Prix contenders. Kel Carruthers was 250cc world champion that year, and the points he earned were split between a four-cylinder four-stroke Benelli and a TD2.

Modern as the TD2 then was, making 44 hp at 10,000 rpm, its points-triggered magneto ignition was technology from the deep past. The cam that opened and closed those points was on the end of the crankshaft, whose continual flexure caused spark scatter that required very cold spark plugs and, in turn, made occasional plug fouling a fact of racing life. Therefore, at Daytona that year, Yamaha tested two more accurate forms of ignition timing, one based on a magnetic sensor and the other employing a radio-frequency trigger. The times they were a-changin’.

How could energetic, accurately timed sparks fail to make an engine run sweetly? Someone quickly figured out the problem.

In Europe, the availability of fast two-stroke production racers sent ripples into Spain, where the first Femsa electronic ignition was in prototype form. By any laboratory test, the new Femsa was an excellent ignition because its sparks were energetic and their timing had far less scatter than a points-triggered magneto. The trouble was, it didn’t work on the racetrack; it misfired chronically.

How could energetic, accurately timed sparks fail to make an engine run sweetly? Someone quickly figured out the problem. When the transfer ports open in a two-stroke cylinder, fresh fuel-air mixture jets in, mixing with the hot, inert residual exhaust gas swirling there. Perfect mixing cannot take place because at 10,000 rpm the whole transfer process takes two thousandths of a second. The result, at any given point in the cylinder—at the spark-plug gap, for example—is a steady procession of packets of as-yet unmixed gas. Here’s a packet of exhaust gas. Next comes a mixing zone, mostly inert exhaust, with a bit of fresh. And here comes a cell of clean fresh mixture. What if the spark duration is so short that the whole spark discharge occurs within a single packet of exhaust gas? That will be a misfire. And in a mixed zone? Probably a misfire as well.

We can imagine the Femsa engineers looking at an oscilloscope screen displaying the spark discharge from a magneto. Magnetos build up energy in an iron magnet pole and wire coils wound onto it. The coils have inductance, which means their discharge takes a while. There on the O-scope screen was the engineers’ answer: a magneto, spinning on their ignition tester, was maintaining an arc at the plug gap for 1,200 microseconds, whereas their own electronic igniter, with its much more intense discharge, was maintaining its arc for just a very few microseconds.

A short-duration spark can easily begin and end in an unignitable zone, producing a misfire.

The magneto’s 1,200 microsecond discharge corresponded to 72 crank degrees. During that whole time, an electric arc was present, flaring between the spark-plug electrodes. That gave enough time for several turbulence cells of cylinder gas to pass through the gap, ensuring ignition every time.

When Accel decided to develop a non-magneto ignition for top-fuel drag cars, it had the very same experience. The mixture in a top fueler’s cylinders is a crazy brew of compressed air, all sizes of cold nitromethane droplets, and whatever vapor that has had time to evaporate from those droplets. A short-duration spark can easily begin and end in an unignitable zone, producing a misfire. Those old-timey Vertex magnetos sticking out of top-fuel engines were there for a good reason: They generated sparks whose long arc duration would find an ignitable zone every time. Accel extended its spark duration until results were good.

When the Femsa electronics were modified to produce longer-duration sparks, their ignitions became a must-have for riders racing the new and affordable Yamaha 250cc and 350cc two-stroke twins. The German Krober and Spanish Motoplat CDIs appeared in the same period. Soon CDI was adopted by Yamaha for its production roadracers. The revolution was under way. Only later would come today’s ability to “map” ignition timing, giving engines best-torque spark timing at all points of their rpm range, leaving behind the bad compromise of fixed timing.