Low Tension Magnetos
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.
Test equipment used:
Gould Model 1604 storage oscilloscope
4 to 1 attenuator (for 400 VDC pulse)
Hewlett Packard color pro plotter
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