Is there a way to tell if I have a single or dual fire ignition? It does pop ever so often. Is that single or dual when that happens?

 

Easiest way is to look at your coil. Single fire ignition system has two wires coming from the ECM; one for each cylinder ( threaded ring style terminal fitting) . Dual fire only has one wire from Control Module and then one sparkplug wire for each cylinder of course.

 

Single fire on OEM was rare until they brought out the EFI bikes. I'm guessing that a 1990 would be dual fire.

 

1990 FLHS here! They certainly were. Would need an after-market ignition system and coil to change over.

 

So what is the advantage of one over the other?

 

The "dual fire" system, also known as "Waste Spark" has been on our bikes for a very long time. It fires both plugs at the same time. When they fire, one cylinder is in the exhaust stroke, so it doesn't matter, and the other is in the compression stroke. A single fire system only fires the cylinder at the compression stroke. A single fire system has more time for the coil to charge between firing, however, a dual fire system is probably good up past 15K RPM, and I really don't see any of our bikes turning that (more than once, briefly, anyway) so it is just fine. My Camry and Corolla have Waste Spark. I redline the Corolla at 7500 for daily driving, but that motor is good to about 9K without too much piston wear. My Lexus has Single Fire ("coil on plug," actually.)

When single fire came out on HD's in the late 80's or early 90's, there was a whole lot of hype associated with it. Dynos at shops were rare then, so was it better? Well, if you just spent a bunch of money on it, you thought it was better. People have since dynoed them and not noticed any difference.

 

Thanks Doc--- That is sort of what I was told by the guy I bought/traded the bike from. He said it causes an occasional "pop" but to install the other system would not add anything---just do away with the "pop". I was not sure if it was the dual or single that I had. I am thinking of buying a used tach, but it will only work with the dual spark and I did not want to throw out money for something that will not work on my bike.

You guys are great. There is a ton of knowledge here.

 

I just hate the idea of wasting anything; esp. a good spark. Seriously, wouldn't the one firing plug have a stronger spark then? I went to Bosch "+4" plugs that have 4 electrodes instead of one for a more dispersed spark. Also no more "popping/backfiring" now with the Crane Hi-4 Single Fire Ignition System since my mechanic recommended it with my top end job.

 

Well, it doesn't really matter with the waste spark. The cylinder under a lot of pressure on the compression stroke has a high resistance seen across the electrode. The cylinder that is the "waste spark," in the exhaust stroke, has a low (relatively) resistance seen on the electrode. Don't forget that your spark plug wires have a considerably amount of resistance built into them as well (or should, especially if you have a radio or EFI). The way it's wired up is as one big coil secondary, and the two spark plug leads are both ends of the coil. It all works out.

I've heard a lot of horror stories about those Crane Hi-4 systems. I'm glad your's is working out, but for a couple years on here there were a bunch of "I got no spark, what's wrong?" threads and a disproportionate number of them were running those Crane systems, which turned out to be the cause. We've had a lot less of those threads since Crane went bankrupt. Also, any improvement you observe is more likely from timing control changes than not wasting the spark.

 

I'm sure the big difference in power and acceleration is the top end job and the midrange cam my mechanic put in. Also the S&S carb really seems more reliable and consistent than that OEM monstrosity. When I first bought the bike used it seemed to backfire consistently so maybe combination of the CV carb and the double sparking ignition. It's smooth, powerful, and really pulls now in the mid range.