Tachometer question

[Troll]

Waste spark has been around for over 100 years. The Drifters are not dual fire, in the sense you are thinking. The 1500s are dual plug, however. This is both how and why it works: A spark coil when energized, is looking to get rid of the energy through discharge. Spark energy will only jump the gap with the least resistance. A cylinder with a burnable mixture inside will ionize and allow a path for the spark to jump, the cylinder on the exhaust stroke essentially does not have a burnable mixture, so no ionization, no spark jump. It's actually a pretty simple system to get a multi cylinder engine to run with only one ignition trigger. In the old days, a dual tower coil was fired by a single set of "points".  Mid year 1978, H-D replaced the points with an electronic module, but the rest of the system dated back to 1911...This is essentially the same kind of system that cars had up until the digital ignition processors took over, and after distributors went away...I had this system on all my H-Ds, as well as on my BSA, and on my GM cars of the 80s-90s...

[John Hopkins]

The wasted spark is not really wasted, it is to burn off any remaining fuel on the exhaust stroke..It makes the engine burn cleaner..or at least that is what some environmentalists would have us believe..

John.

[Bucko]

What's the advantage of a wasted spark system on a V twin?  I guess it makes timing simple (no need to divide cranks rotations to time to the compression stroke - that could be easily done by the ECU), but is that the only reason?  Makes all sorts of sense for a inline 4 - reduces the number of coils required by 2 but that doesn't apply to a v twin.

[scamp62]

Then is the Drifter considered a dual fire ignition ?

[Troll]

Tapping both coils will likely keep the engine from starting at all, since each signal wire is actually the ground for the coil. Most tachs, at least they used to, have an adjustment on them for # of cylinders. When installing a tach on a Harley with single fire ignition, a special filter is required to get a reading from the coil. I'll bet that the waste spark system on an 800, using only 1 coil per cylinder, will signal on both the exhaust and power strokes, so there will be no issues with the tach reading 1/2 rpm...

[scamp62]

A little help from the brain trust out there please !
Installing an electronic tachometer on my  800. The question I have is there is a single engine signal line that is to go to the coil, is going to one coil going to give me a true reading, or should there be a line to both coils
Any advice would be greatly appreciated.