Fault searching
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Bad capacitor
It is very common that a new capacitor don’t last very long.
The capacitors you can buy now is of very poor quality compare the capacitors that was available back in the 60is.
A bad capacitor looks like this on an oscilloscope.
There are a lot of discharges inside the capacitor.
If the input voltage is reduced to 6V then it looks more common.
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But the spark time is too short to ignite the fuel/air mixture. It is only 0.5ms.
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A shortage output from the ignition coil
If a shortage occurs on the high voltage side there is a high frequency AC signal on the oscilloscope but no low frequency as indicating the end of the spark. The reason is that the ignition coil is loaded and has low inductance.
If the high tension cables or spark plug is broken then the figure is looking like this.
Note a low frequency, the same as when the spark time has ended. In this case the ignition coil has an open output and the inductance is high.