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Circuit Two

Batteries, having a definite shelf and usage life, often needed replacement. In areas where these were not readily available engines used a so-called "low tension" magneto to feed the igniter points. No external coil is required in this case because the winding of the magneto itself becomes the inductor and the circuit is simplified, also the IGNITER is normally closed and trips open only when a spark is required. The timing of the magneto and the ignitor is important so that its "peak" output occurs at the igniter "break" time.

From Fairbanks-Morse - Principles Of Magnetos

If a piece of steel is bent into a "U", to make the ordinary "magnet," the space between the ends will be filled with invisible magnetic "lines of force." The magnetism will be stronger at this point than anywhere else about the magnet. If a coil of wire is moved in and out of the space between the poles, or is revolved in this space, an electric current will be generated in the wire, and if the ends of same are separated, a spark will be produced between them.

The MAGNETO is simply a magnet, with a coil of wire revolving between its poles, the coil being provided with a suitable means whereby the current generated is conducted off to the engine igniter, the points of which are alternately opened and closed at the proper time.

A peculiar thing about the Magneto is that the current generated in the wire is strongest at only two points in each revolution of the wire, and to get the best spark the igniter points in the engine must snap when the wire is at one of these points.

Drawings of Magneto

Figs. A, B. C. and D. will make this clear. "NS" represents the ends of a magnet, having soft iron pole pieces for the purpose of concentrating the lines of force near the "armature," which is merely a piece of iron on which the wire is wound, this iron helping to concentrate the magnetism so it will more powerfully affect the wire.

The armature is rotated in the direction of the arrow; and in the position shown in Fig. B the wire begins to be affected by the lines of force, and current is generated. A little beyond B the wire gives up the energy which has been generated in it. Just before the position Fig. C is reached, the rate of change in the lines of force is greatest, and the current coming out of the wire is at its maximum. This is the "PEAK" or highest point of the current wave, shown in Fig. E. As the lines of force tend to follow the metal in the armature, it will be seen that, in the position shown in Fig. C, the lines of force all pass through the iron, and are diverted from passing through the wire, consequently the current drops to zero, as shown at point 2, Fig. E.

Sinewave Theory

When the position shown in Fig. D is reached, the lines of force pass through the wire in an OPPOSITE direction, as it has turned over, and the events just described take place again, this continuing as long as the armature is rotated.

As a matter of fact, owing to the magnetic lag in the armature, the peak of the current is produced with the armature at a point midway between Figs. C and D, but from the foregoing it will be evident that there are two points during each revolution of the armature when the current is at a maximum, and the igniter points in the engine cylinder must SNAP to produce the spark with the Magneto armature at either one or the other of these "peaks."

This necessitates some positive mechanical means of driving the Magneto, such as gears, and does not permit the use of belts, friction wheels, speed governors, or other devices that allow any play or slip.

Suppose for instance that the igniter points should open when the armature was in a position midway between the position shown in Figs. A and B. Now, by reference to Fig. E it is obvious that the current strength will not be at its maximum, and instead of eighty, or whatever the maximum might be, only ten or twenty units would be available, and the spark would be too weak.

No adequate provision has heretofore been made to enable the position of the armature and its relation to the igniter when at the peak to be readily determined. Heretofore this timing had to be done by experts. While some machines are provided with interior marks, to be observed through openings in the casing, which makes it difficult to observe the timing, Sumter Magnetos have a patented method of timing, which enables them to be easily timed by anyone, and the indicator is entirely external, and easily seen with the Magneto in any position.

From the foregoing it will be observed that a Magneto is very simple, but the details of its construction, the method of bringing out the current through non-wearing contacts, the quality and treatment of the steel used in the magnets, so as to insure permanency, etc., are all points that require the most careful consideration.


The manufacturers of the Webster Tri-Polar Oscillator, realizing the seriousness of the ignition problem, set out not only to develop a magneto that would overcome the deficiencies of gas engine ignition, but to make a magneto as near fool proof as possible.

The Webster Tri-Polar Oscillator. Instead of CONTINUOUS rotation, the ARMATURE on this magneto is SNAPPED with the TRIP LEVER and it then returns to its neutral position by the force of the attached springs. Thus the term OSCILLATOR.

Before the Webster was known, it was considered that 90% of engine troubles were found in the ignition. Since the advent of the Webster Tri-Polar Oscillator, these troubles have been overcome. Engine manufacturers have openly testified to the worth of this device by making it standard equipment on their respective lines of engines.

The phrase "Webster Tri-Polar Oscillator" has been made to mean a full ignition equipment ready to place upon an engine. It consists of the magneto, igniter plug bracket, push rod, wedge and journal, and is the first in the history of ignition for internal combustion engines to be successfully furnished as a complete, self-contained device. The Webster Tri-Polar Oscillator was never sold without the unit plug bracket designed especially for the particular make of engine for which it was intended.

In the Webster are found many distinctive and patented features, not to be obtained in any other ignition device or system. Because of its sturdiness and simplicity it has won an enviable reputation as a money and labor saver for the engine manufacturer, dealer and user.

When equipped on an engine the Webster becomes an integral part of it, is positively operated by means of a push rod and runs independent of engine speed. To make a spark, the magneto requires but one twelfth of a revolution, the balance of the time it is at rest; also when the engine cuts out, the magneto remains idle, resulting in minimum wear on all parts.

The Webster Tri-Polar Oscillator gives battery quality conditions for starting and permanent ignition for the life of the engine. This is successfully accomplished only in the Webster, because of its simplicity and many patented features. Eliminating batteries means doing away with long wires, many connections, switches and coils, which have caused many engine problems.

As the Websters windings are stationary, it is free from moving wires, brushes and collector rings, the troublesome parts of all other magnetos. It follows therefore that it is weather, water and oil proof, which is one reason why the Webster is so successful. The reason is readily understood. Where moving wires are employed, it requires a copper collector ring, insulated from the shaft, and a carbon brush, to carry the current from the magneto. Should dirt, water or oil get between the ring and brush, the circuit is broken and the magneto rendered useless.

In the Webster, the points of the electrode are always together except when opened for the fractional part of a second during the operation of the magneto. Therefore as the points are closed most of the time, very little carbon will gather upon them. However, should some particles of carbon collect on the points, the movable electrode is so placed in the igniter that it can be pressed inward, rubbing the points together, thereby cleaning them and making perfect contact.

On each equipment is placed an advance and retard lever for early and late ignition. When starting an engine, if the operator desires to crank it, with the lever in the retarded position, the engine will not "kick back" and hurt him.

One of the most important features of the Webster is the starting lever that is furnished with each magneto. With this lever all the difficult work is removed from starting an engine. All that is required is to first cock the magneto, then prime the engine with a charge of gasoline, pull the flywheel back against compression, and then press down on the starting lever, which causes the magneto to spark and the engine to start.

The Webster Tri-Polar Oscillator is the simplest, most efficient ignition device ever made. It has made possible the high standard of efficiency of the engine to which it is attached.

American Bosch AB33 and AB34 for Fairbanks Morse:

FROM SMOKSTAK® Bosch AB33 Coil Replacement etc.

SUMTER Plugoscillator (Fairbanks Morse & others):

Sumter Magneto

TIMING: Turn the Early/Late eccentric so that the letter E is up, indicating the early spark position. Then rotate the flywheels in the running direction until the Plugoscillator snaps. The SPARK mark on the flywheel should be over the push rod. If not, proceed as follows:

Loosen the clamp nut and adjust the trip finger bracket forward or backward on the exhaust push rod as may be necessary. Then try the timing again to make sure that it is correct. This is the only adjustment that is required on the entire ignition apparatus.

Be sure to retighten the clamp nut so that the trip finger bracket cannot slide on the exhaust rod, and be sure that the knife edges line up. The trip finger engages the trip and break lever in the notch and is tripped by the projection on the break finger moving downward and pushing it from the notch. Make sure that the mounting nut which clamps the Plugoscillator into the engine combustion chamber is kept tight.


Motsinger AutoSparker and Wizzard types:

The two methods of producing a current by mechanical means are by the use of dynamos or magnetos. A dynamo has field magnets of soft iron or mild steel, wound with wire through which circulates the whole (series) or a portion (parallel) of the current generated by the machine. A magneto has field magnets constructed of steel and permanently magnetised, no part of the generated current adding to the magnetism.

The field magnets of a dynamo increase in strength as the current which passes around them increases. As the magnetic strength increases, the voltage of the generated current becomes stronger. As a result, a dynamo is not self-regulating and if run at too high a speed. is liable to be overheated in its effort to furnish a current beyond its capabilities.
AutoSparker Dynamo Drawing of AutoSparker Dynamo and Wizzard Magneto

The Motsinger AutoSparker shown here has been rebuilt and is now in use on my 4HP IHC Famous. The AutoSparker is a self excited dynamo and the Wizzard is a permanent magnet magneto, but both types were used along with external low tension coils in the ignitor circuits of larger engines. The main consideration with these is that timing is accomplished by tripping the ignitor. Since the output is DC (direct current) there is no cause for concern about the timing of the armature vs. the ignitor trip time as compared with AC output from John Deere or Associated magnetos. Another point to consider is that while starting the engine, there may not be enough armature speed to generate full voltage. Therefore, a two position switch is used to allow starting on battery power and then cutting over to generator power after the engine is up to speed.

A small diameter friction pulley can give sufficient speed to ignite the charge when engine is turned slowly as in cranking. After the engine is under motion, the governor on the shaft of the dynamo limits its speed so as not to obtain an excessive voltage. This is accomplished by mounting the dynamo on its base so that it can oscillate on an axis, the small friction wheel making and breaking contact with the engine flywheel. In operation, when normal speed is exceeded, the governor weights fly out and draw the friction wheel away from the flywheel, one spring serving the double purpose of pushing the friction pulley against the flywheel and acting as a tension on the governor. By increasing or diminishing the tension on the governor spring by means of a thumb nut provided for the purpose, the speed of the dynamo may be increased or diminished, which in turn increases or diminishes the volume of current and size of spark. By screwing up on this thumb nut the position of the dynamo is not changed, but the contact of the pulley and tension of the governor are increased. Thus, by adjusting the thumb nut, the size of the spark may be regulated at will.

AutoSparker Dynamo

Associated and John Deere Alternating Current types:

Some Magnetos that are designed for farm engine use do not produce a continuous (DC) electric current. In these, there are only two points in each complete revolution of the armature that it discharges its maximum electric current. It is therefore necessary to time the magneto so that one of these points is reached just at the time the igniter snaps.

IMPORTANT: Before touching the magneto or endeavoring to make any adjustments see that the igniter is set for the proper firing point. Just as the Igniter trips, the correct position of the crank shaft for the "Chore Boy" engine is 15 degrees below inner dead center. This angle varies from 10 to 30 degrees depending on the speed of the engine and the length of the stroke.

CAUTION: Never touch the magneto until you are sure that engine is property timed and the igniter, and igniter points are clean and adjusted. See that the mica insulation is not burned out. It is a good plan to wash the igniter thoroughly in kerosene. After these instructions have been carried out and it is then found necessary to time the magneto, follow the instructions below.

Timing Magneto

Associated Iowa and John Deere

FIRST: Note the direction the magneto gear turns when the engine is running. By referring to the illustration of the magneto, you will note there are two small PUSH BUTTONS, one marked "L" and the other marked "R." If the magneto gear (marked M. G.) turns toward , the letter "L" the push button near letter "L" is to be used. If the gear turns toward the letter "R," the button near "R" is to be used.

SECOND: Stand on the igniter side of the engine, turn the flywheels in the direction the engine runs, stopping instantly when the igniter snaps and do not allow the wheels to turn. CAUTION-Be sure engine is "firing" at proper time, otherwise you are likely to make the engine run worse rather than better.

THIRD: Loosen the screws that hold the magneto to the magneto bracket, then raise the magneto high enough to lift the gears out of mesh.

FOURTH: Then turn the magneto gear in the same direction you found the magneto gear (marked M. G.) running when the engine was turned over. If the magneto gear runs toward the letter "L," press in on the push button near letter "L" and turn the magneto gear until you feel the PUSH BUTTON enter a slot in the inside mechanism of the magneto. Hold this PUSH BUTTON in and lower the magneto so that the gears are again in mesh and tighten the screws that hold the magneto to the bracket.

PLEASE NOTE: If you find the magneto gear (M. G.) turns towards the letter "R," use the PUSH BUTTON near the letter "R" following the same timing instructions as above.

TIMING OLD STYLE MAGNETOS. On some of the old style magnetos the magneto gear was fastened to the magneto shaft by a set screw, only there was no idler gear. In this case it is not necessary to loosen the four screws that hold the magneto to the magneto bracket, but just loosen the screw that holds gear and remove it from the magneto shaft. This will allow you to turn the magneto shaft without raising the magneto from the bracket. Use the same timing instructions as given above. NEXT PAGE: >

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