Magneto Drop
in 2 Stroke Technical Questions
14 years ago
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by Richard Jowsey » 14 years ago
I measured the AC voltage at the tacho tap (grey wire) while toggling the mag switch at 2000 rpm: voltage drops from ~30 down to 3 volts! Although the tacho is supposed to measure waveform frequency, a ten-fold voltage drop could explain the drop in measured RPM. Does anyone know what the "spec" voltage should be, ex-factory, from grey wire (tacho tap) to ground, when open/shorted?
I initially assumed that the rev-counter tap would be in the middle of one of the "charging" coils, so the voltage presented to the tacho would be the same whether or not the winding was shorted (to ground). Apparently this may not be the case.
TIA,
Richard
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by Richard Jowsey » 14 years ago
There were, in fact, two problems. Apparently, the timing was coincidental...
I removed the stator windings, and eventually found a break in the "lighting coil" copper wire near where it's soldered to the yellow wire. The copper wire at the point of breakage was completely embedded in a glob of hot-gun resin, so it probably had a tiny fracture, which gave way over time due to vibration and thermal cycling. Anyway, I soldered up the breakage, and it now works fine. Magneto voltages are good. Battery is charging.
I discovered a 50% tap on the ignition coils, so I tried connecting this to the tacho (instead of the 10% tap) -- now there's no bogus drop of indicated revs when toggling the mag switches. Works for me. :-)
Cheers,
Richard
Thank you said by: Clive Wilton
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by Nick Axworthy » 14 years ago
Nice one Richard....you obviously know your stuff,
Out of interest...like me most of us will be stuck with the odd reading the tach give when doing the mag check....am I correct in thinking its an erroneous reading and I am better going by ear and just listening to the note of the engine for a big drop in RPM to ind a prob.
Nick in the UK.
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by Richard Jowsey » 14 years ago
The Rotax tacho lies sometimes, when the supplied voltage is too low. Unless you want to do some tricky soldering, just listen to the engine note.
Cheers,
Richard
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by bill martin » 14 years ago
All my buddies have the same problem with their two strokes. We have just learned to ignore the tach and listen to the engine. I don't know what the difference is, but I have never had the problem with my 912.
Bill.
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