Cub magneto j4

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Edward1993
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Cub magneto j4

Postby Edward1993 » Thu May 21, 2020 3:49 pm

I just bought a j4 magneto and have no idea how to install it.the distributor was removed from tractor when purchased. I do not have spark to plugs. 1951 farmall cub. New plugs and wires seem to be in good shape. 12v system

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Re: Cub magneto j4

Postby Glen » Thu May 21, 2020 5:50 pm

Hi,
Welcome to the website.

Below is a page from the 1949 Cub operator's manual, telling how to time the magneto to the engine.

http://farmallcub.com/rudi_cub/www.clea ... age-32.jpg

Below is a pic of how the drive gear slots should be turned when the engine is at number 1 cylinder TDC.
Sometimes people take the engine apart, and don't put the gears together in the correct teeth positions.
If it is not correct, you might not be able to time it correctly.

If you meant you have the Battery Ignition unit, it may need the ignition points, or condenser, or coil replaced to have spark. The Battery Ignition unit is a newer unit than the magneto.

If the Cub originally had a Battery Ignition unit, you will need a different ignition switch to use a magneto.
The magneto is not connected to battery power.
The Battery Ignition unit is connected to battery power.
The ignition switch for the magneto grounds the magneto to stop the engine. So there is no battery power in the wire from the switch to the side of the magneto.
Don't connect battery power to a magneto.
The Battery Ignition switch has 2 posts for wires.
The magneto switch has 1 post for a wire.
I would probably repair the Battery Ignition unit so it works, if the Cub has one. :)
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Re: Cub magneto j4

Postby Barnyard » Thu May 21, 2020 6:14 pm

You can run one wire from the ignition switch to the side of the magneto. The other wire from the switch to ground on the dash. However, to start the stractor you will have to push the switch in, and pull out to stop.
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Re: Cub magneto j4

Postby Stanton » Fri May 22, 2020 6:39 am

Edward1993 wrote:I just bought a j4 magneto and have no idea how to install it.the distributor was removed from tractor when purchased. I do not have spark to plugs. 1951 farmall cub. New plugs and wires seem to be in good shape. 12v system


Edward, welcome to the forum.

Some more information about your Cub would be helpful:
  • Did you just buy your Cub? Are you new to Cubs?
  • Did you buy a new replacement J4 magneto or is this a used one?
  • Did you remove the distributor or did the previous owner remove it?

There are many helpful threads in the How To Forum about Ignition/Electrical items, including magnetos. Check out this one on how to time your magneto:
http://farmallcub.com/phpBB2/viewtopic.php?f=140&t=44397
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Re: Cub magneto j4

Postby Cubfriend » Fri May 22, 2020 2:27 pm

Make sure the plug wires are copper core (inside) for a magneto. They don’t like the modern style wires. (Carbon core?)
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Re: Cub magneto j4

Postby SONNY » Sun May 24, 2020 1:46 pm

Did it come with the dist. OR did the seller take it off and keep it? --- IF you have the dist.--put it on and never look back!! Mags are a joke!!! ---I got a half dozen dead ones and the tractors just sit because of them! ----- I did offer to trade them for dist. and no takers,---so that proves to me that mags are not popular!

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Re: Cub magneto j4

Postby Eugene » Sun May 24, 2020 2:55 pm

A photo of your current magneto, distributor, or absence there of would greatly help.
I have an excuse. CRS.

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Re: Cub magneto j4

Postby k hutchins » Mon May 25, 2020 2:40 pm

Not everyone feels that way Sonny. I just bought a new to me '48 and it had a distributor. The first thing l did was post on the vine to try and find a mag to swap it out.
I like the idea of being able to start and run my tractor even if the battery is dead (which happens at times). Can't do that with a distributor.
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Re: Cub magneto j4

Postby SONNY » Wed May 27, 2020 9:56 am

Being unable to hand crank tractors anymore----I GOTTA have battery start stuff! ----Dist is a breeze to work on when needed---mags are expensive and like diesel injection pumps not everybody can afford the equipment to fix them when they dont work. ----speaking of diesel tractors---I got 4 of them sitting that the pumps went out on a few years ago. ---I can fix motors but not injection pumps OR mags! lol!

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Re: Cub magneto j4

Postby Eugene » Wed May 27, 2020 10:09 am

Edward1993 wrote:I just bought a j4 magneto and have no idea how to install it. the distributor was removed from tractor when purchased. I do not have spark to plugs. 1951 farmall cub. New plugs and wires seem to be in good shape. 12v system
Does the engine crank over with the starter? Engine stuck? You need to do compression tests, dry then wet, before you spend any more money on this tractor.

Photos of the current distributor/magneto area.

If the tractor is currently 12 volts, the electrical system has been converted. More photos of the electrical system.
I have an excuse. CRS.


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