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Magneto TroubleShoot
The Ignition Lead - Electrical Behavior
The ignition lead directs the energy wave from the magneto to the spark plug. The lead shielding prevents the radiation of wave energy. Because the lead exhibits reactance, it modifies the energy wave.
Capacitance is the ability to store an electrostatic charge between two conducting plates separated by a dielectric. Lead insulation is called a dielectric meaning it can store electrical energy as an electrostatic charge. An example of electrostatic energy storage in a dielectric is the static electricity stored in a plastic hair comb.  Skytronics
Ignition lead construction
In an ignition lead the conductor is one plate of a capacitor and ground the other. Shielding the
ignition lead increases capacitance by bringing the ground plate closer to the conductor plate. The ignition lead consists of a conductor covered with a dialectic, and then metal braid shielding (another conductor) and then
air (another dielectric) and then the engine (another conductor). Electrically the lead acts as a linear capacitor and has the ability to absorb and store electrical energy. The energy stored in the lead is called distributive capacitance; it is essentially static electricity at rest. The magneto must produce enough energy to charge the distributive capacitance and have enough energy left over to fire the plug.
Ignition lead capacitance increases the energy required to spark the plugs. For example, a Slick coil with 1.5 ampere primary current will fire a 5 millimeter test gap using a three foot ignition lead with the lead's shielding ungrounded. Once we move the ground plate closer by grounding the shielding, the coil wont fire the gap. More primary current is needed to fire the plug with the shielded lead.
Distributive capacitance is released once the plug fires and becomes conductive. The lead discharges when a conducting path is provided between the plates. The ignition lead shielding and one side of the spark plug gap is connected to the engine block. The other side of the spark plug gap connects to the leads inner conductor. Ionization of the spark plug gap completes the circuit between the ignition shielding and the ignition lead's inner conductor.
The rapid rise-time of a capacitive spark erodes the spark plug's negative electrode. The resistor within the spark plug (Champion uses a 5,000 ohm resistor) reduces plug erosion by dissipating some of this energy as heat.
Distributive capacitance can be lowered by:
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Removing the ignition lead's radio shielding.
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Shortening the ignition lead.
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Avoid routing the lead against the engine case.
For all practical purposes lead capacitance is a function of lead
length
If we reduce distributive capacitance (i.e. reduce lead length)
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More reserve magneto energy is available to fire the plug.
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Less spark plug electrode erosion occurs.
A small, thin, weak-looking spark is characteristic of a capacitance spark. The fat, big blue spark
of the Bendix S1200 magneto, is characteristic of an inductive spark.
The ignition lead has inductance. That is, the lead stores energy in a magnetic field and releases this energy when the spark gap ionizes. Lead capacitance and inductance offer a resistance to the flow of current called Ignition lead reactance.
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