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Farmall & IHC Tractors Discussion Forum
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H magneto weak spark

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Dave

06-05-2004 05:05:00




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My h starts when cold and runs fine, but when allowed to idle down after warmup, it will die and not restart. At that point it will then have none or very weak, spark. I just bought this thing but I don't hear the famous click from the impulse coupling when turning enging over. Is there anything in there that tends to wear out and need service?




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El Toro

06-05-2004 08:39:11




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 Re: H magneto weak spark in reply to Dave, 06-05-2004 05:05:00  
Hi Dave, The spring on your impulse coupling may be weak or broken. Try installing a new condenser
they seem to fail after they get warm. I had a magneto that had a weak spark and after installing new points and a condenser, it was still weak. I pulled the mag off again and took
it to an auto electric shop where they had a machine to test mags. They installed the mag on the machine and found the coil weak. They had coils from the gas 1790 engine that was used in
the early M47 and M48 tanks and ground one those
coils to make it fit in the Wico mag.

The spark would bite you pretty good after having a new coil. You may be able to find
another mag on a junker. After I had the coil replaced on my mag I pulled the mag off and kept it in a dry place. A damp location is a killer for coils on a mag. Good Luck, Hal

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Jon

06-05-2004 07:24:03




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 Re: H magneto weak spark in reply to Dave, 06-05-2004 05:05:00  
If your tractor is converted to 12 volts you may not here the impulse snap They stop after they get spinning so fast.Take some light oil like 3 in 1 and oil it liberaly at the oil cup bye the impulse. Better if you can have it turning over while you do it.That lubercates the trip.Let me know what you find out I may be able to help.If you try to take it apart you really need a small puller to get the trip off the right way.So be carefull. JON

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JON

06-05-2004 07:33:32




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 Re: Re: H magneto weak spark in reply to Jon, 06-05-2004 07:24:03  
Also the trip should not have anything to do with your spark. All it does is change the advance to be able to crank it easyer.I would start with the condenser then the coil. JON



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Dave

06-05-2004 07:56:49




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 Re: Re: Re: H magneto weak spark in reply to JON, 06-05-2004 07:33:32  
It has been converted to 12V and the trip is liberally lubricated. I agree with your hunch about the condenser or coil. My understanding is that as condensers age they may become sensetive to high temperatures, but, I have noticed that the post upon which the kill switch connects remains grounded even when the switch is opened. This is the case always, no matter if the points are closed or opened.

Should this be so, or is this indicating a faulty condenser or coil?

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tom braun

06-12-2004 16:37:02




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 Re: Re: Re: Re: H magneto weak spark in reply to Dave, 06-05-2004 07:56:49  
About a week ago my H stopped after running for about 3 to 4 hours straight. I could not get it restarted. I replaced the points condensor, coil rotor and cap but still no spark any ideas...



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Bob M

06-05-2004 08:26:34




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 Re: Re: Re: Re: H magneto weak spark in reply to Dave, 06-05-2004 07:56:49  
Dave - It's entirely possible on 12 volts, cranking the motor spins the mag fast enough to deactivate the impulse. However by turning the crankshaft by hand you should get a noticable "snap" from the mag every 1/2 crank revolution.

Not a bad idea to replace the mag condenser (and also the points while you are at it). They can deteriorate with age and may become temperature sensitive. Same deal with the coil - though at $50 or so for a new one you may want to save replacing it for last!

Incidentally it's normal for the grounding post to appear grounded. The current path is thru the coil primary when the points are open. The actual resistance then is only a fraction of an ohm - hard to distinguish from a dead short to ground with most VOMs and DVMs.

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Bob

06-05-2004 08:19:07




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 Re: Re: Re: Re: H magneto weak spark in reply to Dave, 06-05-2004 07:56:49  
One side of the coil primary is hooked to ground, and the other to the terminal you mention. Is has relatively low resistance across it, and that's the "short" you mention.



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Bob

06-05-2004 07:50:05




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 Re: Re: Re: H magneto weak spark in reply to JON, 06-05-2004 07:33:32  
The impulse in fact has a VERY LARGE effect on spark. The magneto rotor is held stationary, and then released, and the rapid movement imparted by the spring GREATLY increases the spark.



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Wayne Swenson

06-05-2004 06:18:38




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 Re: H magneto weak spark in reply to Dave, 06-05-2004 05:05:00  
The impulse coupling may be stuck and need cleaning.
Rotate the engine so the mag rotor points to #1 spark terminal, then take the mag off for inspection & service.



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