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F4 magneto questions

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Doug Steinbach

01-03-2017 02:08:43

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After mostly working out of the area for a year I went out to crank the F14. So it's been sitting for a year. No spark.

Here's what I've done.

Pulled the mag so I could check the points and all the obvious things. I've determined the coil, condenser, and all that stuff is good. I actually have another mag with a cracked case that I swapped all the parts around on and it all works fine on that one. Magnets are both good. Pretty much everything that can be swapped around I did.

The only obvious difference in the 2 is the snap of the impulse coupling. The mag with the cracked case is much tighter than the one off the tractor. The mag off the tractor is a rebuilt one that I bought a few years ago and the impulse coupling never was as hard to snap as over as the original. I always suspected that was because I had the spring turned 1 or more turns tighter than the rebuilt one. ??? I've pretty much ruled out the impulse coupling as the problem as the tractor has a starter on it and it actually will crank without the impulse coupling snapping if you stay on the switch and continuously crank the engine over. At the same time it started fine with the hand crank.

Since I've never had the mag completely apart I proceeded to disassemble the mag with the cracked case just to make sure how it came apart so that I didn't destroy the rebuilt mag in the process.

So here is the question.

Is it possible the rotor shaft is bad? I was sort of expecting to see rust on it, or even rust inside the case, but neither is true. It's all new and shiny looking. The mag with the cracked case however is rusty, but it seems to generate a nice spark.

The only thing I can figure is the tractor is mourning the loss of The Cowboys to the Eagles............

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Charlie M

01-03-2017 04:52:57

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 Re: F4 magneto questions in reply to Doug Steinbach, 01-03-2017 02:08:43  
When you crank the tractor can you actually hear the impulse coupling working. Sometimes it doesn't on my F14 and a small tap on the side of the mag gets it working again. Did any moisture get into the cap or the mag it self. Concerning the rotor turning, you can turn the mag by hand if you have it off the tractor and see if the rotor turns. You can also see the gears if you take off the cover.

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Doug Steinbach

01-03-2017 05:21:47

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 Re: F4 magneto questions in reply to Charlie M, 01-03-2017 04:52:57  

Yes the impulse definitely works. Checking the cap for moisture was the first thing I did. I've actually encountered that problem before.

Right now I have the mag completely disassembled. I may not have used the correct terminology, but the rotor shaft I'm referring too is the shaft that goes through the mag. I'm inclined to think the shaft or the field windings (for lack of a better description) inside the housing are shorted out. Is that possible?

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Charlie M

01-03-2017 06:59:57

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 Re: F4 magneto questions in reply to Doug Steinbach, 01-03-2017 05:21:47  
I'm sure not an expert on these things so hopefully some guys that know more will chip in here. Is there any moisture inside the body of the mag. Mine sat out in the rain one time without the gaskets for the cover and filled up with water. Even with gaskets I keep a plastic bag on the 2 mags that I have to keep them out of the rain. Is your carbon rods all riding on the rotor. How about carbon build up on the rotor. I don't know if something can short out in side unless something has dropped into the body or a build up of water. If you know the parts like points and condenser are good then this about all the ideas I have to look at.If you're not sure on a moisture problem maybe leave the mag in the house in a warm spot for a few days with the cover off and see it things change.

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Doug Steinbach

01-03-2017 07:20:53

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 Re: F4 magneto questions in reply to Charlie M, 01-03-2017 06:59:57  
Obviously I'm not an expert on mags, much less anything else. The only reason I know all the obvious parts are good is that I put them on another mag and it works fine.

I guess rather than saying shorted out, I should have said grounded out. Which I know the mechanism to ground out the points is working because I can physically see it.

I'm not getting any spark right off the coil. The secondary lead out wire has a wire that jumps spark to the where the coil essentially mounts. On the other mag with the cover off I can actually see the spark there, but not on this mag. Even when I change all the parts out to the other mag I still get spark there, which is why I'm certain all the parts are good.

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