I was 20 years old and had never set ignition timing on an engine. A college friend had finished assembling his early 1950s long-stroke Triumph 500cc twin and was confidently expecting me to time its gear-driven magneto. I didn't know how.

That magneto was about the size of a short brick. One end of it generated electrical pulses, concentrating their energy in the form of internal magnetic fields. The other end contained a rotating switch—the contact breaker—spinning inside a cam ring with two internal-facing bumps. A phenolic plastic “foot” on the contact breaker was moved by the bumps, opening the switch. Gearing between engine crankshaft and magneto was two to one; the mag completed one revolution for every two by the crank.

At the instant the switch opened, generated current flowing in the magneto was cut off and the sudden collapse of the magnetic field it was generating induced a high-voltage pulse in a winding of fine wire. This pulse, arriving at the spark-plug’s gap through the ignition wire, produced a steeply increasing electric field across that gap. If all was well, random free electrons that happened to exist in that gap were accelerated by the electric field, gaining enough energy to, in turn, knock electrons off of gas molecules in the gap. This became a chain reaction, allowing an avalanche of electrons to flow across the gap. The vast number of electron-to-gas molecule collisions transferred this energy to the gas, heating it explosively (that’s the “snap” you hear when a spark jumps a plug gap, and it is also the explosive crash when lightning discharges close by). If that gap contains fuel vapor as well as air, the spark discharge may very well ignite it; riding a combustion-powered motorcycle absolutely depends on it!

The trouble is, that event had to be timed to occur at the right time. If it happened too early, the engine might kick back during starting—no electric starter on those early twins—and would lose power because the fuel-air mixture would burn to peak pressure too early, causing the engine to fight itself as its pistons tried to compress that too-early combustion.

With ruler and file, I cut visible grooves across the spoke, down its length, 1/16 of an inch apart. This was now my timing gauge.

If the spark were timed significantly too late, peak combustion pressure would occur after the piston had descended too far on its power stroke, causing significant loss of peak pressure because it was happening in a combustion space already considerably expanded by piston movement. Another effect was what a do-it-yourself friend had seen on an ancient car he had mistimed: The exhaust manifold became red hot because the late combustion, instead of delivering peak energy to the descending piston, blew most of it out the exhaust valves and into the manifold.

These threatening facts raced through my head as the time approached when I had undertaken to be Mr. Wizard and time the ignition.

To the rescue came a copy of J.B. Nicholson's great classic, Modern Motorcycle Mechanics. The correct ignition timing was at the point that the piston, rising on compression, was just 5/16 of an inch or roughly 8mm before top dead center. The wise old book advised the practical mechanic to fetch a three-cornered file (I had one), an old wire spoke (check), and a ruler (I was supposed to be a student, so, yes, I had a ruler). With ruler and file, I cut visible grooves across the spoke, down its length, 1/16 of an inch apart. This was now my timing gauge.

Modern Motorcycle Mechanics
Described by the publisher as "one of the first comprehensive texts regarding the operation and maintenance of motorcycles of the day," the 766-page seventh edition of J.B. Nicholson's classic Modern Motorcycle Mechanics is available here.Courtesy of Modern Motorcycle Mechanics

With the ignition switch off, spark plugs removed from the engine, and cover off the contact-breaker end of the magneto, I put the graduated spoke vertically into the inclined spark-plug hole until it touched the piston, positioned just before top center on its compression stroke. I could now measure piston position by counting file marks on that spoke.

With a wrench on the end of the crankshaft, I rotated the engine to top center, the point at which the spoke neither rises nor falls but is between. I noted the file mark on the spoke that was next to some obvious feature, like an edge of the spark-plug hole. This was top dead center. I then rotated the engine backward, counting marks on the spoke as the piston descended until I got to 5/16 of an inch before top center. This was the point at which the contact breaker must just be opening.

Ideally that contact opening would be indicated on a resistance meter wired across the contacts, but in that simple, anyone-can-do-it world, contact opening was detected by placing a strip of thin cellophane cut from a cigarette package between the points then exerting a slight tug on it with your fingers.

The desired result was that the cellophane should be released by the points opening just as the rising piston reached 5/16 of an inch before top center, as indicated by the filed spoke gauge.

As I recall, the drive gear on the magneto fitted onto a taper, so the drill was to have the gear loose on the shaft while one person held the magneto at the position in which its contacts were just opening. The other person rotated the crankshaft to bring the piston to 5/16 of an inch before top center. At that point, a “sharp rap with a small hammer” would seat the gear on the tapered shaft, after which its fixing nut could be tightened.

Some adroitness required but eventually it was done, and I checked it several times. I put the cellophane between the points with the piston in mid-stroke, then slowly rotated the engine forward, pulling on the cellophane and watching the spoke gauge to see when the piston had reached 5/16 of an inch before top center. Yes! It was good.

“Is that it?” asked the friend.

“Yes, it’s timed.”

The Triumph started and ran. The result felt good, but I knew I was as fake as the Wizard of Oz. It was actually J.B. Nicholson who timed that Triumph engine.

I kept that spoke for years.