Deep Dive on wasted spark ignition failure modes...

[Cossack_38]

I had an ignition failure that prevented my '38 U big twin from starting, until I found it. Once corrected, it started instantly, but leaves me puzzled... Most of us know the basics of the "wasted spark" coil; it is designed to fire both plugs every time. I have seen one plug get fouled (shorted to ground) on numerous occasions, and the bike starts and runs on one cylinder.

Since the bike wouldn't pop at all, I checked voltage at the coil (normal), cleaned the points, and tried swapping in a new condenser. No luck. I suspected bad coil, so I measured resistance of the coil primary winding (about 3.5 Ohms), and secondary winding (about 16K Ohms). All pretty normal. I flicked the points open with ignition on, looking for a spark at the points. Intermittent at best.

I eventually found some corrosion at the very end of the front spark plug cable, where it goes in the coil. It looked like it developed an internal gap that the coil couldn't jump. I trimmed about 1/4" off the cable end to get back to fresh wire strands, put it back together, and it started immediately.

I don't get why one bad plug wire (open circuit) prevents starting... I expected the front plug not to fire at all, but why would it have weak, intermittent (or no) spark at the rear plug with a bad front wire? Can anyone make sense of this for me?

[aumick10]

The wasted spark system doesn't actually fire both plugs at once. The coil generated voltage goes through one spark plug to ground (motor). The voltage then travels to the other park plug and back to the coil. One spark is always from the center electrode to the earth tang. The other plug goes opposite, from the tang to the center, If one plug is shorted, there is still a full circuit so the second plug will fire. If one plug is open, no circuit exists so neither plug will fire. This the one reason i use single fire ignition on my later bikes.
There is a way to make both plugs fire together and the same direction but i am not aware of any systems using this. Effectively two separate single coils in parallel.

[grease monkey]

Great to know someone is doing this work. I have one of those mags and a Mitsubishi copy. Do you know what the Japanese copy is from ? My first thought was Rikuo IMG_6229.jpg

[duffeycycles]

Isn't normal primary resistance on 6 volt coil 1.5 ohm ?

[Cossack_38]

duffeycycles, I assume you are correct. My U has a Cycle Electrics generator and is converted to 12 volts.