Intermittent ignition cutoff

[JimCT]

It all started last winter plowing snow. Tractor starts instantly... no problems. After 10 minutes, sometimes sooner, the engine cuts out briefly then resumes normal running. Sometimes it does it only once per hour or so of running, sometimes more frequently. Fuel flow is fine... no starvation and the carb is good... it just behaves as if I'd momentarily pushed in the kill switch then pulled out out again. Sometimes the engine dies entirely and after cranking (starter motor) for about 5-10 seconds, engine starts & runs smoothly. Tractor has mag. ignition. There is no lack of power when she's running.

What I've done to try to correct problem: New points, plugs, condenser, mag coil... all of which had no effect on the problem... not worse, not better. Thinking it may be the kill switch itself, I disconnected that and the problem remained.

The ignition system is dead-simple on these girls but I'm stumped. Any one out there in interwebnet land have any ideas? If you're nearby and come up with the fix it's worth a beer of your brand. We've had the Cub since '52 and this is the first time she's had this sort of a problem. She's no show girl; still works hauling a trailer & plowing snow.

[Eugene]

This one is going to be hard to diagnose. Sounds like it's an ignition/magneto problem.

Would be easier to solve the problem if the tractor just quit and wouldn't start.

[offrink]

Double check the wires. One could be shorting out.

[Smokeycub]

Agree with Eugene and offrink. Does it cut out after the engine has warmed up or act up at any time. If it does it after warming up then the mag coil could be going bad (i.e. a n internal break or short) but normally a coil going bad will show other signs, like a weak spark. By your description it could be an short or a broken wire anywhere and with a mag ignition there's not much to check but can be a bugger to locate. One place to check is where the lead to the points goes through the housing, sometimes they will short out there and can be intermittent. I'm sure you'll find it. Good luck.

[JimCT]

Agree, if it had just quit that would be a lot easier. Will check the lead to the points. The mag coil is new so that's out of the equation. Haven't replaced the dist. cap so will do that. Have wiggled wires to see if there is an iffy short to ground but nothing found. Will report back.

[Barnyard]

Double check the wire to the mag and make sure it doesn't have a bare spot that might rub against the tractor during operating. If it rubs anything it will cause the mag to shut down.

[Don McCombs]

The mag coil is new so that's out of the equation.

Not necessarily! Check to see where it's manufactured.

[JimCT]

Concerning the wire to the mag from the kill switch, I disconnected that and the problem remained. Regarding the mag coil, after I replaced it with the new one (manufactured God knows where) the problem was the same. I fully agree with your comments... somewhere there is something shorting. Will tear down the whole system again and double-check all wires, insulating grommets and examine everything. Something is breaking down somewhere after she's warmed up a bit... which does indeed point a finger at the coil. Will pick up another one and see what happens.

[Eugene]

Before you purchase another magneto coil, convert the magneto to an external battery powered coil.

If you have an extra coil on hand or can borrow it from another tractor, the test should be at about zero out of pocket dollars.

[JimCT]

VICTORY! it was indeed a short.

Inside the J4 housing where the leads from the mag coil and the condenser connect to the housing shorting terminal, the crimp insulator on the coil wire connector had slipped back and the bare crimp was occasionally shorting to ground. In the process of this final tear-down I found hairline cracks in the bakelite coil cover so that was replaced. She's all buttoned-up now & back to work. She's been with us for 62 years now.