Magneto and spark (or lack of)

[forrest allen]

51 Case SC. I have spark on the number 1 plug but nothing on the others. Using inline tester. New plug wires. Cleaned inside of the terminals to ensure contact on the plug wires. Cap is old but looks good. Cleaned the contact points inside the cap. Tried the wire from #1 in the other 3 holes to see if it was the wires but still no spark. I am stumped as to why I'd only have spark on one wire. Thoughts?

 

[teddy52food]

Bad rotor or bad cap. Does the coil spark on all 4?

 

[forrest allen]

They are not all firing. Only get spark on number 1.

 

[forrest allen]

I guess I may not be understanding your question too. No spark through the plug wires but testing the coil is something I don't know how to do. This doesn't have the coil wire like a distributor/coil does so I am lost on testing it.

 

[jd man wis]

Make sure that the rotor is spinning with the mag shavft. Some of the mags had a pin that went thew the hole in the mag shaft that drives the rotor. Sounds to me that has happen on yours.

 

[Earl-IL]

What brand of Mag ??

 

[ShakerPrairie]

Forrest you could take a pencil/pen eraser block and clean the inside (and outside) of the cap. Also clean the face of the rotor where it contacts the cap brushes.
Another worthwhile project is to clean all of the electrical contact places in the mag. Such as where the points and condenser touch the mag when screwed into place. And also the actual electrical connections for the points and condenser.
With good clean connections it will give you all the spark it is capable of.
You do have to be gentle and not break anything. That stuff is old and parts are scarce.
I don't recall the point gap, but it isn't much. Something in the neighborhood of .010" to .012"
I am assuming that you have a Case magneto.

 

[forrest allen]

Thanks. I think I found my problem.Missing teeth on the rotor made it stand still on number 1.

 

[forrest allen]

How do I find the model of this magneto so I can get the correct rotor? It's a Case with an eagle emblem but I find no numbers except the patent info.

 

[tractorguy2]

That is a Case model 4JMA magneto. There is a guy selling reproduction rotors on ebay. Unfortunately they do not come with the gear. The gear will be very hard to find. They are only available as NOS. They rarely turn up on ebay. You might have to look into having a new gear 3d printed in plastic. Or find one of those guys that do CAD metal work and have a gear laser cut from aluminum.

 

[forrest allen]

Thanks. I've found several 4jma magnetos used but condition unknown and a rebuilt for $350. I just might find a 3d guy and try that.

 

[forrest allen]

I found a source for the gear but I don't know how to get the old one off of the rotor. Any guidance?

 

[dandeere]

Hi Forest That gear looks newer and I just wanted to say to make sure the gear that drives it has good bearings. I had a Wisconsin v4 that I had the mag redone then tore out the new gear because the drive gear could bind it up I pulled my hair out I didn’t even have on that one Good luck. Dan

 

[forrest allen]

Thanks, I'll look at that too