J4 magneto repair shop

[baskintow]

How can you get in touch with Rick Spivey about a problem with a J4 magneto on 1951 Cub. It runs for fine for about 2 minutes and shuts down. I have replaced coil, points, and condenser, as well as cap and coil cover, also plug wires.

[Crimson Tim]

You probably need to have 2 posts to your name, but then you should be able to pm him directly.

Are you sure it’s a spark issue? When I’ve had issues that match your symptoms, it was a fuel flow problem. Rust partially blocking the sediment bowl screen or the carb inlet screen or needle valve in the carb.

[Barnyard]

Take the plug out of the side of the carb and see if you are getting a good fuel flow.

[baskintow]

Thanks, When it shuts down it shuts down and without any warning. you can reach down and it fires back up . It shuts down so quick it backfires. Just like cutting the switch off.

[Eugene]

Are you sure it’s a spark issue? When I’ve had issues that match your symptoms, it was a fuel flow problem. Rust partially blocking the sediment bowl screen or the carb inlet screen or needle valve in the carb.

Concur, sounds like a fuel flow problem.

[BigBill]

When it shuts down either pull the drain plug on the bottom of the carb bowl or pull the tubing off the carb. I had the same problem in my int154. But it would start right back up until finally didn’t restart. I took off the carb hose and no gas was flowing. I took off the gas cap still no gas. I blew into the fuel line as hard as I could and heard a pop in the gas tank. I haven’t had that problem ever since. It was a pebble or a nut in the gas tank.

[Rick Spivey]

With those symptoms described, and the work already done, I concur that this sounded like a gas flow issue to me. The only other experience I have had similar was a loose connection that grounded out my spark. But if yours is fairly consistent about the 2 minutes of running before shutting off, it points to gas flow. That is about how long a full carburetor will run once you shut the gas off, when at low idle.

[Stanton]

Thanks, When it shuts down it shuts down and without any warning. you can reach down and it fires back up . It shuts down so quick it backfires. Just like cutting the switch off.

Let us know what you discover...

[Glen]

Hi,
New electrical parts can be defective.
You might have a defective condenser, that can cause the spark to shut off.
Possible the coil could be defective.

The ignition switch could be causing it to shut off, I don't know if it has an original, or replacement switch. Try wiggling it when the engine is running, and see if it makes the engine stop, or if it feels loose. If it is old, it could be corroded inside.
The insulation on the wire from the magneto to the switch has to be good. It could short and stop the engine if the insulation was bad and the wire was contacting metal.

[baskintow]

Have replaced total of three condenser and three coils total. put another wire and switch to kill engine. before it would skip misfire when it took a notion, Had magneto rebuilt and this problem occurs. Also complete wiring harness replaced about a year ago. I will check fuel flow but it not acting like running out of fuel and it don't matter about the throttle setting either. Thanks for the suggestions.

[Rick Spivey]

The insulator around the stud that protrudes through the mag body might have a crack or missing piece. That will allow it to ground intermittently, which can give symptoms as described. The rebuilder may not have noticed if they didn't look specifically for this, or it could have cracked since then. Put a keen eye on that one if you go any further with the magneto. The parts are available through TM and/or Steiner, possibly others.

[tmays]

When you say you replaced three coils, did the problem disappear for any length of time? And were these new coils?

[baskintow]

Yes sir they were all new coils. After i replaced the first coil, the tractor ran for about 4 hours without a problem. Next day started the same problem.

[tmays]

Yes sir they were all new coils. After i replaced the first coil, the tractor ran for about 4 hours without a problem. Next day started the same problem.

Make sure you’re using plug wires that are copper core and not the carbon ones(or whatever that stuff is). Also, non resistor plugs. And that point gap and plug gap are correct. Mags like the old fashioned stuff.

[Don McCombs]

Have you checked the fuel flow, as was recommended?