Low Tension Magnetos

| March/April 1994

57401 Ray Center Rd Ray, Michigan 48096

Ever wonder just what low tension really means as far as voltage goes? Here are the results of a brief study of a Webster tri-polar oscillator magneto that I conducted. It was a model 'AK' two magnet type.

The process began because of problems encountered with my father's new (old) 'Little Jumbo.' It would run for a short time, then die. Cranking produced only an occasional sputter. It seemed to be getting fuel, so the magneto or ignitor were suspect. It turned out that the ignitor contacts were not always closing. (No spark if the contacts are not closed at the beginning of a cycle.) The shaft through the ignitor housing was binding and the return spring wasn't doing its job.

While the magneto/ignitor assembly was removed from the engine I conducted a test to determine what voltage it takes to produce an adequate spark at the ignitor contacts.

To begin, I charged the magnets. Then, using a storage oscilloscope and plotter arrangement, a trace was made of a single activation of the magneto/ignitor assembly. As can be seen in Figure No. 1, if the contacts are open at the beginning of a cycle, the level is approximately a minus twenty volts direct current (-20VDC). Figure No. 2 is the trace of a points closed cycle. A level of approximately (-) 400 VDC was observed. Note the changes in voltage and time scales on figures No. 1 and No. 2. In both cases the armature was rotated approximately twenty degrees (20°) and released in a trip action. The (-) 400 VDC level produces a healthy blue spark that is easily seen.

After reassembly and a little priming, that 'music to the ear' sound was heard once again.


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