since 1940

Dead aircraft magneto, inoperative magneto, rough magneto, roughness

2. Ignition filter,See Checking the Aircraft Magneto  filter or magneto capacitor
3. "E" gap, See egap.htm for an explanation of E gap

4. "P" lead grounding.

5. Rough left magneto and not able to time left magneto on Lycoming 540 engines, inspect magneto adapter bearing for failure

6. Carbon Arc Tracking inside magneto - The spark takes the path of least resistance to ground. We want the spark to occur across the spark plug gap but if the resistance here is higher than at some other point in the high voltage circuit (possibly because of worn out spark plugs or a dirty or mis-timed magneto) the spark may occur somewhere else resulting in a rough running engine or loss of engine power.

High voltage found it easier to jump to ground inside the magneto rather than at the spark plug gap - at high power settings! Magneto gave a good mag check but when full power was applied resulted in low takeoff power. As engine power goes up the voltage required to jump the gap at the spark plug increases.  A mag check at low power settings does not adequately stress the high-voltage circuit enough to uncover problems that will occur at take-off power.

The spark plug terminal should be clean. If the terminal screw appears burnt then suspect that the harness contact spring is bad and needs replacement or that the resister inside the spark plug is bad and the plug requires replacement. When you service spark plugs spend as much time on the terminal well as you do on the firing end. Do not use contact cleaners as they may leave deposits that break-down into corrosive compounds. Clean with MEK (Methyl Ethyl Ketone) Acetone or Alcohol.

A common problem is the "P" lead wire. If this wire is chafed so that it intermittently grounds, it intermittently turns the magneto off. To field check the magneto condenser or ignition filter see "Checking the ignition filter or magneto condenser". Also check to make sure the "P" lead is attached to the correct starter switch position, i.e. left mag to left mag position.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.