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Allis Chalmers Discussion Forum

ODD problem with FM J magneto

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04-28-2008 16:32:36

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I hadve a magneto that has a very hot spark on 2,3,4 BUT not on number one lead. Checked the tower and they all ohm to 0 reistance. Is there a way to check /repair this problem wihtout having to buy a new cap?

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05-01-2008 20:49:51

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 Re: ODD problem with FM J magneto in reply to loveshisallis, 04-28-2008 16:32:36  
What method are you using to determine you have spark or not at a paticular cylinder number? If you are checking it at the plug I would suspect the wire to be the issue. Have you Ohm'd the wire? If the cap shows all posts to have no resistance and 3 of four are sparking properly there isn't much chance the the problem is in the Mag. However, there is a chance in a thousand that a misalignment of internal parts due to bearing wear could cause one not to fire but the problem would more likely be erratic and show up at different posts at different times.I have witnessed similar problems in other configurations that turned out to be defective coils however, again, the order of failure jumped about to different posts. If the cap is used check for carbon trails leading away from the affected post inside of the cap. "ohm around" inside of the cap from the the affected post and see if you can get a connection to show up that may be leading the spark away from the post. Good luck!

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04-30-2008 19:43:55

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 Re: ODD problem with FM J magneto in reply to loveshisallis, 04-28-2008 16:32:36  
I would have to say that the problem is between the rotor and your cap. you could try and clean the contacts or replace the rotor but I can't think of any way to check for that without taking the mag apart Good luck Ed

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04-28-2008 23:39:24

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 Re: ODD problem with FM J magneto in reply to 180doug, 04-28-2008 16:32:36  
Rotor contact through the cap through a copper core wire had better read 0 ohms. If it were infinity, the problem would be solved! Either buy a new cap or wire as needed.

If you check each post to the other you should get infinity on each. If that is also true the problem should be a bad plug.

If no #1 spark when shorting to block (no plug) check carefully with a magnifying glass for a poor rotor connection at the wiper in that position.

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04-28-2008 21:04:52

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 Re: ODD problem with FM J magneto in reply to loveshisallis, 04-28-2008 16:32:36  
They certainly DO NOT all "ohm" to zero resistance! You obviously are confused between the meaning of "zero Ohms" and "Infinity"!

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