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Tractor Talk Discussion Forum

need some magneto help please

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gary cook jr

01-12-2005 22:01:29




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Im in the process of restoring a 42 farmall a with a magneto .I have the tractor up on stands with the engine torn down ,with no contact to the ground at all . My question is ,when i put the magneto on and crank the hand starter should i not get a spark ?I have no fire what so ever , and was wandering if anyone has any knowledge of where to start ?I opened the front of the magneto and the points were not opening at all , so i fixed that and still nothing .Could it be the condenser ?This is my first complete tare down and rebuild ,so im really a green horn !! any ideas would be appreciated . Gary C

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1945 A

01-13-2005 09:12:00




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 Re: need some magneto help please in reply to gary cook jr, 01-12-2005 22:01:29  
There's a bit of information in one of the other posts that may end up confusing you.
While it is true that the impulse "firing" of the mag is a minor issue when testing for fire, it will be VERY IMPORTANT (impulse operating correctly) when you eventually try to start your engine, since cranking speed (whether by starter or hand) will be much slower than actual engine rpm, even at idle. The purpose of the impulse is to provide enough speed as the rotor passes by the steel stationary shoes to build up enough voltage in the coil to "fire".
If the impulse doesn't work correctly, you could have the magneto firing great if you spun it with an impact (as suggested), but it still wouldn't fire while cranking, since it (magneto) wouldn't be turning fast enough. You would then be forced to tow the tractor in order to get enough rpm for the magneto to fire.

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xragman

01-13-2005 08:03:38




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 Re: need some magneto help please in reply to gary cook jr, 01-12-2005 22:01:29  
check on john deere page. plenty of good advice there.



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JT

01-13-2005 06:53:48




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 Re: need some magneto help please in reply to gary cook jr, 01-12-2005 22:01:29  
How are you checking for fire? Do you hear a click in the magneto when it is turning over? this will not make any differance when looking for fire, but you do need the impulse working to start it later, it retards the timing to make it easier to start. The easiest way to check it is to mark everything, take off mag, take a 3/8" air impact and spin it backwards, while grounding plug against case. it will show you if you have fire. If nothing, take a voa meter and see if you have coninuiuty between the point contacts when they are closed, if nothing, then you need to replace or clean points and reset to .017 point gap. If this still does not get you fire, then replace points and condensor. If that does not do it, then you may need a coil. You might also stick a small screwdrive in and touch the rotor area and make sure th magnet has not lost its charge, not very likely, but is alwasy possible.

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JT

01-13-2005 06:55:02




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 Re: need some magneto help please in reply to JT, 01-13-2005 06:53:48  
I spoke wrong, when turning backward, hold wire coming out of coil about 1/4" from case, not spark plug.if you have fire then, it will fire plugs.



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1945A

01-13-2005 06:24:13




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 Re: need some magneto help please in reply to gary cook jr, 01-12-2005 22:01:29  
Not sure of what type magneto you have, and unclear about how you"re testing it (other than cranking it over), but, you didn"t mention anything about having your wires and plugs attached to the magneto, and that has to happen. Also, the base of the plug has to make contact with the frame or engine block to provide a ground for the mag.
The other thought, is that you should here a "snap" coming from the mag as you crank it over--- that"s the sound of the impulse coupling spring winding up and relesing, which creates enough speed for the mag to fire. If you"re not hearing the impulse operation, that"s a place to start.
I hope I haven"t confused you more, but, you"re not very clear on your testing procedure...

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