I forgot to say, you either have a hit and miss governed JD single cylinder engine (did JD make such?) or a 2 cylinder tractor. I grew up on a JD 420 with the odd fire design (40, 420,430 all the same I believe). The distributer cap sure is odd looking with the two wires over on one side and 2 blanks on the other.

I never thought about the horizontal 2 cylinder JD's before, is that what you have? They would be the same as the side mount ponies, pistons would have to move together to have an even fire design. Is that where that distinctive sound comes from?

The 14A -15A D8s and 17A D7s and the 18 & 19A D9s had the even firing engine with the throws on the crank both together but the later starting engine used on them and other ind. engines went to an odd firing engine with the throws a 180 D. apart.

John Deere made quite a few "make and break" engines back in the day. That's where the term "Johnny Popper" originated.

It is a horizontal type engine with tons of torque but no tons in weight!
I take it to the Fair every year and pull a 5500 lb class and always spin out.
It is hard to describe the firing pulse but it fires both cylinders one right after the other, then spends the rest of the time letting the flywheel do the work.
I figured on the opposed twins mag discussion that my "Food for Thouught" was probably hopeless.
If it was my Magneto, I would have it rebuilt as I am out of ideas like the rest of you.
Though I would change the wires and sparkplug BRANDS first.
New Champions are notorious junk, vintage ones are Fine like I run in the John Deere.
I would try any other brand of plug before I condemmed the Magneto.

I finally got around to going out an installing my rebuilt Wico Mag. From the first noticable firing problem, it had taken the crack in the magneto cap about 4 years to get to the point of only firing on one cylinder. It was a slow death for the mag. I had an Eisman mag also that I could have had rebuilt but it would have cost twice as much to rebuild as the Wico mag. The rebuilder said that Eisman mag parts are out of site for costs, and the Wico mag fits the D4 7U pony so much better. (read as room to work around it). Anyway, the old cap had a very small crack in it that only could be seen by wearing your reading glasses while looking at it under bright lights. Now the pony motor fires on both cylinders and the pony motor hasn't ran this good in years. It cost me about $150 to have the mag completely rebuilt, and it was money well spent. On the cat I stayed out tonight pushing over burden at our pit until it was dark o'thirty. The D4 7U sure does feel good pushing a full blade of dirt. Tomorrow it wll be pushing up stump piles.

SSsssteamer, Whats the wico model# that fits the d4 pony? Who's your rebuilder, if you don't mind my asking?? I'm just wondering if my eismann has a weak spark under load,any way to test??? Thanks, quinq

My Magneto rebuilder is: Industrial Electric & Machine 1030 C Street, Bellingham, WA 98225 tel (360)733-4920 Labor on the rebuild was only $85. The rebuilder said that my Wico mag is still available off his shelf for a replacement on the Cat pony motors. I do not remember what model number it is. As hard as it is to get to, I don't want to take my mag off again to find the model number on it. The replacement magneto cap part number is: 3-5013 The spark can be measured by it's color, girth, and the distance that it will jump. You want a blue spark that should be able to jump at least a 3/16" gap from the wire end to ground. If your spark will only jump the distance of your spark plug gap, then it is too weak to reliably fire the plug under compresssion every time. A yellow spark is too weak to rely on too. Before my mag rebuild, my pony was running only on one cylinder and poorly at that. With the weak spark that I did have, my pony motor should not have even started at all.

Try baking the cap in the oven at 200F and then try it asap. If this gives improvment boil and bake it several times.A debilitating crack is impossible to see usually...Jim