CJ750 toolbox

Magneto for a 12V Chang? - by Ross Kowalski

Really, all a motorcycle needs is a magneto and a lighting coil. They are reliable, so reliable that most piston driven aircraft use them. (Of course they have a backup as well.) They are also simple and clean, no wiring, just bolt them on and time them. I chose a Wico X because it is one of the most common and by extension, inexpensive magnetos available as well as having lots of replacement parts and service info. When I started looking for a magneto, I found a guy selling them as part of an estate. The magnetos were freshly and professionally rebuilt. I got a very very good deal on that magneto.

A little about magnetos. The X does not have a spark advance, manual or automatic. It has an impulse. Impulses are a mechanical sort of spring assembly that retard the first few sparks as well as giving a slightly stronger first spark. The retard is adjustable. There are old tractor resources on the Internet with all kinds of info on magnetos if you are interested.

So where am I on the magneto project? Some recent talk on the Chang Jiang Experience forum about ignitions got the magneto out of retirement and even photographed.

A good choice for a magneto would be a Fairbanks Morse unit from a little industrial twin. (The FM is used on HD sportsters.) Of course they cannot simply bolt one on as we can because of weird V-twin timing cam.

I tested the first version which worked fine, but without the impulse, I had to manually retard and advance the timing. It ran fine though the timing adjustment will need to be made better. I have a couple of systems in mind, but haven't put anything to metal yet.

A Wico X magneto grafted on to a CJ M1M/M1S point housing.
This is a closeup of the nameplate on the magneto, though almost any magneto would work.
I removed the impulse and drive gear from the mag. I don't even know what engine it was for but it was gear driven.
The is the modified timer shaft. I ground out the center tower for clearance.
The first system I tried had a rubber coupler disk that fit over the two pegs visible on the original unmodified Wico impulse plate.
Here is that system. Notice how far the CJ timer plate sticks out of the bottom of the magneto.
I turned down the point housing at a freind's shop.
I then cut slots in the hardened impulse plate with a angle grinder, took off the little pegs, and shortened the Wico input shaft a centimeter or so.
The result is much shorter.