Sometimes You found that your engine is running rough ,after several test you come to know that problem is due to magneto, than it may be caused by Carbon arc tracking between distributor block electrodes & nothing to do with capacitor and contact breaker points.Carbon arc tracking creates a high voltage short circuit causing the wrong or multiple spark plug to fire.In the late stage of tracking ,burnt carbon track appear going from one terminal to another terminal, at earlier stage ,carbon tracking is not visible so the block looks ok.
Inside the magneto, conducting carbon dust from the distributor gear brush combines with moisture (or oil from a leaking oil seal) to form a wet conducting contaminate (carbon conducts electricity). This conducting film, along with the large electrical potential difference between distributor block electrodes, causes a current of a few milliamps to flow through the moist layer causing slight heating. This heating leads to the formation of an occasional very narrow "dry gap" in the conducting film. Concentrating most of the voltage across the tiny dry gap resulting in tiny arcs. Such dry gaps and arcs occurs randomly over the surface in an effect known as scintillation (the original Bendix magnetos were called "Bendix Scintilla"). According to research done by The Electrical Distribution Industry in Carbon Arc Tracking:
Inside the magneto, conducting carbon dust from the distributor gear brush combines with moisture (or oil from a leaking oil seal) to form a wet conducting contaminate (carbon conducts electricity). This conducting film, along with the large electrical potential difference between distributor block electrodes, causes a current of a few milliamps to flow through the moist layer causing slight heating. This heating leads to the formation of an occasional very narrow "dry gap" in the conducting film. Concentrating most of the voltage across the tiny dry gap resulting in tiny arcs. Such dry gaps and arcs occurs randomly over the surface in an effect known as scintillation (the original Bendix magnetos were called "Bendix Scintilla"). According to research done by The Electrical Distribution Industry in Carbon Arc Tracking:
These tiny arcs have a temperature around 1000 degrees Celsius and so cause intense heating of the insulation surface on a micro area basis sufficient to pyrolyse (chemically decompose by the action of heat) any organic polymer.
Each tiny arc deposits a small spot of carbon. Over time a complete "carbon track" path forms to enable flashover. By the time you can see the "carbon track" your engine has been firing the wrong plugs at the wrong time; a situation that could result in preignition and severe engine damage.
The best preventative is to make sure you have the latest design distributor block, replace the distributor block regardless of its appearance, and perform regular magneto preventative maintenance.
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