Ever wonder why Webster put that bolt on the igniter movable electrode (anvil), as seen sticking out on the left of Figure 1? Why didn’t Webster put a bump on the casting or the push-finger (hammer) seen in Figure 2? The bolt is seemingly too long if its only function is to compensate for wear. Actually, the bolt is for timing adjustment. Timing is the location of the piston, or flywheel when the igniter trips. Generally, the igniter should trip 5- to 8-degrees per 100rpm before top dead center (BTDC). There is, however, a second timing adjustment even more important than BTDC. The second timing requirement is the rotational location of the magneto armature when the igniter trips.
All low-tension magnetos go through an oscillation as the rotor is turned. As it turns, the magneto will reach a maximum positive output, fall to zero, climb to a maximum negative output, fall to zero again and then start over. As the magneto turns, the current coming out will build and then fall back to zero. As the magneto continues to turn, current will build up, traveling into the magneto before falling to zero again.
Figure 3 shows the output of a Webster Type M (after the springs were removed) spinning at 500rpm. The output has a positive peak (with current flowing out of the magneto), then goes through zero to a negative peak (with current flowing into the magneto). For this Webster or any rotary magneto, it is important that the magneto is at one of those peaks when the igniter trips. Tripping on a negative vs. a positive peak simply means the electrons will jump left to right across the igniter rather than right to left. For a standard rotary magneto, getting the output to peak at the igniter trip time is accomplished by properly meshing the magneto gear to the cam gear or the crankshaft gear. Getting a Webster magneto output to peak when the igniter trips is accomplished with the bolt in Figure 1.
Websters don’t normally spin, rather the trip mechanism rotates the armature 30 to 40 degrees before the igniter trips. When the igniter trips, the two large springs accelerate the armature back toward the neutral position (see Figure 4). As the acceleration continues, the rate the armature is turning increases and the magneto output rises. When the armature reaches the neutral position, it has a lot of momentum and over-shoots. The springs now decelerate the armature, causing the magneto output to go to zero and reverse its direction, heading back to the neutral position. The magneto output then builds in the negative direction. The magneto output goes into a damped oscillation, or vibration, until magnetic drag and friction bring it to a halt. Figure 5 is the output current (when tripped) of the magneto seen in Figure 4. The first positive peak is 0.96 Amp followed by a negative peak of 0.28 Amp.
Why is this important?
The energy of a spark is exactly the amount of energy stored in the Webster coils when the igniter trips. The energy in that spark is E=1/2 X L X I². L, the inductance, is how much the coils act like coils. L, like the 1/2, is a number that never changes. The current “I” at the time the igniter trips is the only variable in how hot a spark is. But, it’s not just I, but I² (or I X I). Thus, as adjustments are made to the timing bolt, it is only necessary to see if the current is getting larger at the moment the igniter trips. If, by changing the timing, the current is doubled at the time the igniter trips, the spark energy goes up 4X. Timing is therefore critical; small changes in the coils current at the time the igniter trips create large changes in how hot the spark is. It should be pointed out that engine collectors use the term “hotter spark” to mean stronger not higher temperature. In this article, the physics term “energy” and the street term “hot” are interchangeable.
With the bracket and igniter reattached, the bolt on the movable electrode (anvil) was set to just touch the push finger (hammer). Figure 6 shows the output current in yellow and the igniter points in blue. When the blue line jumps up, the igniter points have popped open. When the hammer first strikes the anvil, the points pop open against their spring and initiate the spark. The points stay open as long as the armature is traveling in the positive direction. When the armature has reversed, the points begin to bounce open and closed.
After the igniter trips, the output is at 0.5 Amp when the points pop open, creating a spark. Although a nice spark was observed, had the opening of the points been a bit later (to the right in the scan), a much stronger spark would have occurred.
In Figure 7, the timing bolt is backed off the push finger 3/4 of a turn, or about 0.040-inch. The Webster coils now have 0.88 Amp of current and are near their peak when the igniter trips.
In Figure 8, the bolt is backed off the push finger another 3/4 of a turn, for a total of 1-1/2 turns, or 0.083-inch. Now when the igniter trips, the magneto current is 0.96 Amp and at its peak value. A consistent spark is observed across the points at 1-1/2 turns.
The very slight shift in timing, about 1 ms (0.001 second), and the large change in current can be seen by comparing Figure 6 to Figure 8. Remembering that it is only necessary to look at I² when the igniter pops open, what does 1-1/2 turns of the timing bolt really mean? At 1-1/2 turns, the current was 0.96 Amp, 0.96 X 0.96 = 0.92; at 3/4 of a turn, the current was 0.88 Amp, 0.88 X 0.88 = 0.77; at hammer touching the anvil, the current was 0.5 Amp, 0.5 X 0.5 = 0.25. It may be hard to believe, but for this magneto, if the timing bolt is backed off 1-1/2 turns, the spark will be 3.6 times hotter, resulting in a 3.6X longer duration spark. A 3.6X more energetic spark will have the same color and brightness as the original but will significantly improve combustion.
It is well-known that the original Webster instructions set the movable electrode touching the push-finger when the armature was in the neutral position. That setting is safe and gives a good spark, but in the early 1900s, there were no AC Amp meters, AC Volt meters, oscilloscopes or computers. Today, with modern instruments, a better setting can be found. Well-known and well-respected magneto guy Mitch Malcolm (of Lightning Magneto) has always set Websters with approximately a 1/16-inch gap. I have personally found that all Websters on my bench function better with a 1/16- to 3/32-inch gap.
Engine enthusiast Dr. David Cave is a retired electrical engineer and regular contributor to Gas Engine Magazine. Contact him at JDEngines@cox.net
