I had a very interesting telephone conversation Friday with a fellow from "Aircraft Magneto Service" of Bainbridge Island, Washington (www.aircraftmagnetoservice.calls.net). It was prompted by the 350 hour inspection of the dual pressurized Slick magnetos on my TSIO-550E.
Parts are needed for both mags, but one mag shows signs of having more trapped moisture than the other one. In particular, the mag with the tach sensor has more rusty discoloration than the one without the tach sensor. Apparently this is because the mag needs to be able to vent the pressurization air to get a steady air exchange and when a mag's aluminum plug normally containing the .020 vent hole is replaced with the tach sensor -- the .020 hole is gone. The person at Aircraft Magneto Service recommended drilling an .020 vent hole in the magneto body to restore the venting. Actually, he recommended drilling the hole in the tach sensor body....but i'm afraid i'll drill through something and ruin the sensor. Has anybody heard of or done this?
Then we started talking about spark plugs: I mentioned my plane has Champion fine wire plugs. He said all plugs have an internal resistor, and for some reason the resistance of many Champion plugs seems to increase as the plug ages. Once the plug has been cleaned, the resistance can be measured with a standard ohm meter between the center electrode and the connection inside the plug barrel -- all new plugs should have a resistance somewhere around 800 - 1200 ohms. If the resistance ever measures above 5000 ohms, the plug should not be used. The fellow told me plugs having too high of a resistance can/will cause misfiring and potential problems with the magneto coil and or plug harness. A spark plug bomb tester might show this plug as good, so the ohm meter test should always be performed every time the plugs are serviced.
Unlike an Autolite plug, a Champion spark plug resistor can be changed. Look inside the barrel of a Champion plug and notice the slotted screw. Remove the screw and the resistor will fall out: it looks like a slug of carbon about 1/2 inch long and maybe 1/8 inch diameter. You can put the ohm meter on either end of this slug and measure the resistance -- this is the element that needs to measure from about 800 to 5000 ohms. The resistor is apparently common among all Champion plugs so he recommended visiting your pile of old plugs; removing the resistors; and saving the resistors that measure "good". When you run across an otherwise servicable plug whose resistance is too high you can change its resistor for one from the "good" pile. He says this can cure a host of otherwise mysterious "rough running" problems and forestall future problems.
He said some Reno racers remove the resistor and replace it with a piece of copper wire to get a much hotter spark for racing. Says it wears out the spark plugs very quickly -- but does make a performance difference. Is this true?
-isaac
ES-P N7842K
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