J4 Magneto

Fri Sep 03, 2010 5:11 pm

Hi folks, new on here and don't really know how to do this but I will give it a good try.
I recently purchased a 1950 cub S/N: 112047. It is not in the best of shape paint wise but I don't think that the mechanics has any serious problems. Now for the question. I think I have a J4 magneto. At least the s/n starts with J4. I am unable to get any spark. I have torn it down, but not all the way. I refiled the points & cleaned it. Not real sure how to check the coil. The primary of the coil reads around 2-3 ohms. The secondary is a little over 10 kohms. I am thinking that it is good. Now I am looking at the impulse drive. Isn't it suppose to sort of compress the spring and then trip. Currently the only resistance I feel is from the power of the magnet. I was going to take the impulse drive off but it requires tool number ..... Can anyone advise me on this and what is the thread size of the puller so that I can make one. Thanks for any help that I can get.


Fri Sep 03, 2010 5:25 pm

Go here http://www.cleancomputes.com/Cub/Blue%2 ... index.html and look under magnetos --- not all j4 but the info is great.

Re: J4 Magneto

Fri Sep 03, 2010 5:58 pm

Thanks Charlses. I did go there but the pictures do leave a little to be desired. It is a great place and I am glad to get what info I did but I still need a little more info.

Re: J4 Magneto

Fri Sep 03, 2010 6:41 pm

If when you turn it over by hand there is no resistance I would say that the impulse is defective. Make sure when you do turn it over the mag is in the proper upright position. With no impulse the mag is not turning fast enough to generate a spark. Another thing you could try is turning it over with a drill. That would spin it fast enough so that it would fire without the impulse.

Re: J4 Magneto

Fri Sep 03, 2010 7:03 pm

The impulse puller is available from Earl here on the forum. Send a Private Message to him and he can fix you up. His user name is Earl.

Re: J4 Magneto

Fri Sep 03, 2010 7:19 pm

Thanks guys you have been a big help. I will reassemble it and try the drill motor and I also will try to get in touch with Earl on the puller.

Re: J4 Magneto

Fri Sep 03, 2010 8:05 pm

Do you get no resistance which ever way you turn it by hand :?: Looking from the drive end if you turn it clock wise you should run up against the spring.

Re: J4 Magneto

Fri Sep 03, 2010 8:17 pm

Donny M,
Thanks for the reply.
It hits a stop when I turn it clockwise. When I turn it counter clockwise I feel only the resistance of the magnet. Cecil suggested that I run it with a drill. Well Cecil is correct. When I turn it faster it really puts out a good blue spark and makes a very good crack. I am just glad I wasn't holding the wire. I have send a private note to Earl for a tool or info on a tool to pull the impulse drive to check and see if there is anything that might be worn or broken. The spring and everything I can see is ok. One other thing I did do is cook the coil in the oven. I read on one forum, maybe this one, putting the coil in the oven and heat it to 250 and then let it cool off in the oven. I heated it to 200 and let it cool.

Re: J4 Magneto

Fri Sep 03, 2010 8:24 pm

You're turning it the wrong way. It needs to turn clockwise looking at the back of it. The resistance you're feeling is the impulse. If your hands are strong enough, keep turning it clockwise and it will trip. Do this only with some kind of spark gap to ground or you will burn the coil. Cecil's info is flawed.
Last edited by Donny M on Fri Sep 03, 2010 8:29 pm, edited 1 time in total.

Re: J4 Magneto

Fri Sep 03, 2010 8:29 pm

Just to clarify, the mag will generate a spark whatever way you turn it. The impulse will only work turning it clockwise looking at the drive.

Re: J4 Magneto

Fri Sep 03, 2010 11:46 pm




Re: J4 Magneto

Sat Sep 04, 2010 3:49 pm

sounds to me like its ok time the engine and put it on bet it will work

Re: J4 Magneto

Sun Sep 05, 2010 9:13 am

The impulse puller is a 9/16' - 28 thread, IF I REMEMBER CORRECTLY.....

As someone who broke one doing it another way, I don't recommend ANY other way. However, yours does not appear to need disassembly. Don't go looking for trouble. If you are getting a hot blue spark, the problem is not in your mag. Put it back on the tractor, in time, and look elsewhere.

edit - corrected my thread size above to read 16, not '6
Last edited by Rick Spivey on Mon Sep 06, 2010 5:37 pm, edited 1 time in total.

Re: J4 Magneto

Sun Sep 05, 2010 8:12 pm

I would turn it clockwise even if you have to put it in a vice, and make sure it is tripping. tape a spark plug to the mag and hook the coil wire to it. You can turn it by hand if you have good strength in your hands. If not put it in a vice and us a wrench on it.


Re: J4 Magneto

Sun Sep 05, 2010 9:38 pm

AS boss said

turn it clockwise even if you have to put it in a vise.

there could be something loose inside that will let it turn ccw and lock up clockwise.
by putting it on the tractor you could do some serious damage to the timing and idler gears