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39 B Magneto Problem

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04-18-2003 08:24:18

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Magneto just overhauled. Sparks well when tested in shop. Generally no spark when rotor is turned slowly, as when engine is cranked. Pulling to start didn't work. Magneto does not seem to give snapping sound when turned by hand. Any suggestions?

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Steve in N.J.

04-19-2003 20:27:34

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 Re: 39 B Magneto Problem in reply to PaulT, 04-18-2003 08:24:18  
Dave, On a magneto fired machine, you need the snap coupler in order to initially start the engine. The problem with a mag at slow speed is the spark is weak when the engine is first being turned over. The impulse coupling or "snap coupler" takes care of this problem by retarding the spark and providing a hot spark neccessary for starting. If the coupler isn't working, it could be from a number of small problems. Quite often the torsion spring gets blamed, but it isn't always the spring that causes the problem. Sometimes one of the pawls on the pawl plate may hang up and not swing out from centrifical force to contact the pawl stop to stop the rotor to allow the torsion spring to wind up. Usually this is caused by dirt or grime that collected in the coupler over the years. If the mag was rebuilt, that should be part of the rebuild. It might have been overlooked. In any case, you can drag that tractor around all day, and if the coupler isn't working, she's not gonna start! I rebuild and service mags. If you need any help, drop me a line and I can walk ya through it. If you find any broken parts, I have parts in stock. Just need to know what model F/M mag it is. Good Luck!
Steve/B&B Custom Circuits

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04-18-2003 17:40:32

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 Re: 39 B Magneto Problem in reply to PaulT, 04-18-2003 08:24:18  
You may want to check the impulse the red neck way---tap on the mag with the crank -- yes they can break if you forget the common sence thing.

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Rod (NH)

04-18-2003 11:15:03

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 Re: 39 B Magneto Problem in reply to PaulT, 04-18-2003 08:24:18  
Hi Paul,

It sure sounds like there is something wrong with the function of the impulse coupling. I am not sure what you mean by "sparks well when tested in the shop". If you had your mag overhauled by a shop for money, they should not have returned it to you (or taken your money) without a functioning impulse coupling. The impulse coupling is unidirectional. When you turn it by hand (mag off engine) are you sure you are turning it in the right direction? If it doesn't snap in either direction, the coupling is the problem. Watch it when you do this...mags can give a healthy bite. Make sure you don't drop it on your toes third party image.

third party image Rod

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04-18-2003 12:32:39

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 Re: Re: 39 B Magneto Problem in reply to Rod (NH), 04-18-2003 11:15:03  
Thanks, Rod.

My magneto was repaired in December but I made no attempt to install it because of cold weather. Turning the rotor by hand, no spark could be found. I recently returned the mag to the shop where they turned the magneto at engine speed to check the spark from each outlet. Under these conditions the magneto seemed to work well. I suspect some problem with the impulse coupling. At the shop they talked about the possibility of a broken spring. They were able to turn the magneto by hand to identify position number 1. The shop foreman didn't think they had any replacement springs. Seeing no options, I installed the magneto but found no spark either while cranking by hand or when towing with another tractor. Paul

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04-18-2003 20:09:41

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 Re: Re: Re: 39 B Magneto Problem in reply to PaulT, 04-18-2003 12:32:39  

The mag needs the impulse to fire correctly. The impulse is set to lag 30 degrees while the timing is advanced 30 degrees, resulting in fire at TDC. If the impulse is broken, there's no way for it to be timed right.

Take the impulse drive apart from the rear. They are simple enough to figure out and if the spring is broken, you'll see that right away.

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