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LenNH
03-21-2008 07:42:15
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Re: Farmall F20 FINALLY enroute Nebraska to Illinois in reply to doug in illinois, 03-20-2008 19:50:58
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The mag should give a spark off a plug wire held near the block (give it a little space to jump).
Some very minor things can result in no spark:
1) dirty points (use a file, but NOT emery--it can short out the points, as I remember from the days long-ago when there were points in everything) 2) points too close together--not opening enough because the rubbing block has worn down. More important: 1) bad condenser (I think that can be checked with a continuity tester--unhook the wire and touch the continuity tester to the wire AND the body of the condenser; if you get a continuity reading, the condenser is shorted).
I HAVEN'T WORKED ON THIS SORT OF THING FOR YEARS, SO IF I'M WRONG, SOMEBODY PLEASE WRITE IN AND CORRECT ME. Politely, of course. 2) Weak magnet. I don't know if there is a good test for this on the magneto. If you pull off the magnet and it still attracts iron, it's probably all right. JUST BE SURE YOU DON'T CHANGE THE POLARITY OF THE MAGNET--MARK THE OUTSIDE AND PUT IT BACK THE SAME WAY. Charging magnets requires
a "magnet charger," which I think is something that you can make if you have to, but you'll have to find out how. Somewhere, someplace, who knows where, I probably have something in an old book on how to do that. If that is of interest,
drop me a line by e-mail and I promise to go into my lonely basement and look through some of the old repair books. Mags are just generators with points and a distributor attached, and have pretty much the same problems as generators and distributors. There is another unlikely problem with a mag, and that is a shorted armature. But...if you get a spark straight off, or after cleaning and adjusting the points (.013" with rubbing block on high point of cam) and checking condenser, you're probably in business. The impulse coupling has to work right. Magnetos don't put out much juice at cranking speeds, and the impulse gives the armature a spin to raise the voltage. The impulse pawl could be dry (there is an oil cup on the F-4), and of course it can be worn enough that it doesn't engage. You can hear it drop into the notch with a little clinking noise. If it doesn't drop, DON'T crank with the magneto
on. The impulse holds the mag until the piston is over top dead-center, but without the impulse being engaged, the engine will fire before top dead center and you'll get 55 cubic inches kicking back and trying to break your arm or your thumb (the entire engine has 221 cubic inches). I hope some of this is helpful. There is always the possibility that you've done all this stuff a dozen times and could write a book for those of us who haven't done any of these things for ages. I had two F-20s between 1980 and 1995, but they've both gone off to people who have the space to keep them and the time to maintain them.
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