I discussed the cylinder 3 issue on the Yamaha XJ Owners list. I first wanted to verify that I was not making any assumptions about the actual firing order of the coils, whether the coils do indeed fire alternately, as opposed to sequentially. In other words, if the firing order was something like 1-4-3-2, it would need to fire the two coils in a left-left-right-right sequence, but with 1-2-4-3, it would be left-right-left-right. The EDIS module definitely expects to fire left-right-left-right and if the bike was expecting left-left-right-right, I would need to swap some plug wires around to account for the “change” in coil firing order. However, with all that was suggested on the list and my own understanding of things, I think the coils are firing in the proper order. Besides, it did run on three cylinders at one point.

Another suggestion was that the EDIS system may not work well with the higher impedence stock coils. One lister wrote:

What does the EDIS system expect the coil primary resistance to be? I think that the stock coils are about 3 ohms. When I looked up some replacement coils for an EDIS system from Accel, the primary resistance was 0.5 ohms. The higher resistance of the stock coils could cause the EDIS ignition to not produce enough current to fire the coils consistantly.

Intuitively, I would think that since EDIS module grounds the coil connection that it would power a 3 ohm coil as well as a 0.5 ohm coil. Upon analysis, the suggestion does have merit. Ohms law dictates that a 0.5 ohm coil would pull 28A from 14V, but the supply fuse for the coils and the module is a 15A. Yes, in brief pulses like the ignition system uses, the fuse could probably tolerate nearly double it’s rating, but it seems unlikely that they would do it that way. More likely, they are limiting the available current with an internal series resistor. That series resistor plus the already much lower current draw of a 3 ohm coil could combine to prevent the 3 ohm coil from getting enough current to make a good spark.

So, I stopped worrying about it until I could get a Ford coil to work with.

I stopped at a pull-a-part place on the way home Monday night. They close at 5:30; I arrived at 5:15 with warnings that I only had about 10 minutes. I found the coil on the very first car I walked up to. Four little bolts and two snips later, I’m at the counter laying down $27 for the coil, connector and plug wires. On the road home by 5:25.

So, I clean it up and put a compatible connector on it. I connected one plug wire and put a handy sparkplug (out of the Sportster trike) in it and pulled the fuel pump fuse. I grounded the plug on the engine and cranked. I got a teeny little unimpressive spark and an unhealthy dose of dashed hope.

Then I remembered that these sorts of coils like both plugs connected. The high voltage circuit is from one coil secondary lead to the plug, across the gap, through the block to the other plug, across that gap and back to the other secondary lead. So, I dug around and found another suitable spark plug. Lucky for me the Sportster is a twin. I connected both plugs to the 1/4 coil outputs and grounded both plugs to the engine. This time, when I cranked it, I got a nice fat blue sparks with clearly visible orange plasma. *AND* about every third spark or so, a 15mm long lightning bolt jumping BETWEEN the two spark plugs, as in from one plug’s electrode to the other plug’s electrode.

Faith is restored.

I have found that “Yamaha” is apparently Japanese for “hidden relays” and I will need to relocate another relay, the function of which I have yet to ascertain, one that once resided between the two stock coils. With the mystery relay out of the way, the Ford coil fits very nicely in the space where the stock coils were, even with all four plug wires attached.

On the car, the coil towers “faced” upward, looking like a square distributor at one end of the engine. I have them facing downward. It looks like it was intended to go there.

The remaining work will be wiring, both to the EDIS module and to the plugs. The Ford wires, besides being way too long, have molded on plug ends, I would need to replace these ends, which wouldn’t be too bad a job. They need to be radically shorter anyway, probably none longer that 30cm or so. I’ll have to get the screw on contact tips for my plugs.

The stock XJ550 plug caps appear to have 10K resistors in them, but the monster spark was off resistor wires, so I think I can use some sort of crimp on plug terminals and not worry about resistor caps. Hopefully, I can find a pair of 90 degree caps and a pair of some lesser angle, but straight connectors will work for all four if I must.