I’ve got an early 50 cub with the J4 magneto. I bought it non-running and have been working on getting it going. I’ve got the mag to put out good spark but it seems that the spring loaded snap mechanism in the front of the mag will miss and cause a cylinder to misfire quite often, enough to not keep it running. I’ve taken it apart and can’t find anything obviously wrong with it. Is the magneto just wore out? Thanks in advance.
J4 Magneto problems
- Thread starter Zandr61
- Start date
JohnMihevic
Well-known member
I assume, from your description, that you have started the engine and it is running. If this is so, your problem is not in the mag. The snap coupling is only in the picture for starting. Centrifugal force removes it from starting above about 200-250 RPM. I have gotten my 1948 Cub to idle down very slow and periodically I can hear the mag coupling "kick in". The engine does not miss a beat. The only thing the coupling does when starting is to keep the spark retarded by 13 degrees.
Your intermittent miss, when the engine is running, is caused by another problem.
Agree with John's comments. Here are a couple more thoughts. As stated, at normal engine idle speed the impulse coupling should be disengaged with the magneto simply in continuous rotation at engine speed. Possibly there is something wrong in the coupling that is allowing the pawl to occasionally lock in on the stop pin at idle speed, causing the 13 degree timing lag to jump in at random. I don't think that is a likely failure of the coupling. Even if it was doing that, I doubt that would be enough of a disturbance to keep an otherwise healthy engine from continuing to run. That leaves my other thought, you may be trying to get the engine to idle at to slow a speed. It should be set at 500 rpm. If you don't have a way to directly measure that, just turn it up a bit to see if it helps.
I have sent a few mags to https://rebuiltmags.com/ He will rebuild yours or use it as a core return and sell you one off the shelf that was already rebuilt.
staninlowerAL
501 Club
There are some forum members who specialize in working on the mags, you might ask if anyone is willing to take on your project (or bring it to a Cubfest). JMHO Stan
Dale Finch
501 Club
I'm not sure where you live, but in case you are not aware, there's a cubfest coming up in Bogalusa, LA March 26-28 as well as one in Harrison, OH May 28-30.
At both of them, there will likely be someone who can assist in diagnosing and rebuilding your magneto. You should seriously consider attending... many fine folks who can help with pretty much anything cub related, along with great food and camaraderie!!
At both of them, there will likely be someone who can assist in diagnosing and rebuilding your magneto. You should seriously consider attending... many fine folks who can help with pretty much anything cub related, along with great food and camaraderie!!