Hi Larry,
Do you have any data on the number of hours on the magnetos since they were cracked open, inspected and had the worn internal parts renewed.
There are recommendations to send magnetos to shops for this type of work every 500 hours.
An A & P can do this inspection and repair provided he or she has the correct special tools, but there is a what seems to be a glitch.
I know Cliff Orcutt, the A & P that started Aircraft Magneto Service in Stevensville, MT. I did a story on how his company overhauls magnetos.
One of the things I learned is that the overhaul instructions mandate that rebuilt mags be tested for what’s called the coming in speed. This is the rpm that the magneto can generate a hot enough spark to jump across a prescribed gap to a ground electrode. This test requires a specialized magneto test bench.
I don’t know if the coming in speed test is required during a 500 hour inspection, so since I don’t have the equipment to do a coming in speed test I don’t do 500 hour inspections of customer magnetos.
If the magneto time since overhaul or 500 hour inspection is less than 500 hours, then I would check for the sound of a working impulse coupling.
An impulse coupling is a device that is attached to the drive shaft of a magneto. This device is designed to do two things: 1) retard the spark from the normal timing of 22 to 25 degrees before Top Dead Center (TDC) during the compression stroke (the timing that is correct for normal operation) to around 5 degrees before top dead center.
2) cause the magneto drive shaft to spin rapidly at the 5 degree timing. This generates the hotter spark needed for starting that would be generated at the normal starter cranking speed.
Recently, a local customer was having trouble starting. We discovered a worn and ineffective impulse coupling by: a) pulling the propeller through by hand in the normal direction of rotation (after making sure the magnetos were grounded–key switch in OFF position–and listening for a sharp and distinctive clank type noise. We didn’t hear what we identify as the normal impulse coupling “release” sound so we pulled the magneto and discovered a very worn impulse coupling. Once a new impulse coupling was installed, the starting problems disappeared.
Another way to test the impulse coupling operation is to remove all the top spark plugs (or bottom plugs-whichever is easier, then selecting a plug that is fired by the impulse coupling magneto (typically the left) and holding the threaded portion of the plug against a metal part of the engine, pull the prop through in the normal direction of rotation. In this test the magneto is “hot” (not grounded) so best practice is to pull the prop through by hand. If the impulse coupling and magneto are in good shape it will be easy to see a nice yellowish white spark jump across the gap from the spark plug center electrode to the ground electrode.
If the spark does not appear to be yellowish white, then you need to explore further.
Did the impulse coupling generate a loudish clang noise? If not, it may need replacing
Did the impulse coupling make a loud clang noise but the spark across the plug look weak, or was not hot enough to jump the gap. Then it’s time to get a 500 hour inspection on the magneto.
The other possibility is that the magnetos were incorrectly timed during the annual inspection.
Please let me know what you find.
Steve
