[quote=Blaster;7662770]No i am not wrong, I was talking early pre-evo style engines and of coarse the connecting rods are connected to the crank in the same place. I am sure they fixed the timing thing(firing each cylinder on its own TDC) with the Evo as well as the mis-alignment of the pushrods. Sorry I got off the original question as to single vs. dual fire just giving some history on the dual fire

 

[QUOTE=fchava;7662946]I understood you the first time. No need to be sorry since my main area of question was "how" the "old" dual fire worked without interfering with the intake stroke. You pretty much illuminated that part for me. I would assume that the front cylinder would fire somewhere around 15 degrees or so before TDC anyhow but I had never heard of one firing at 45 degrees before TDC which it would have to if it were to stay out of the back cylinder's intake stroke.
Now I'm just curious about why the tachs need an adapter for single fire since it would seem to me that the coil pulse going to one cylinder with single fire would count the same as the coil pulse going to both cylinders with the old dual fire ?? Either way it seems you are getting one pulse fire per rev (approx).

 

[quote=R_W_B;7663061]When I was at AMI and my teacher told me how the Harleys ignition fired it really threw me for a loop. I guess that since the fuel/air mixture hasnt begun to be compressed since the valves are still open the air/fuel mixture isnt as combustible. But occasionally they do backfire through the intake usually upon startup.
Now the reason a tach needs a single fire adapter is that the coil is being charged twice at a time. Once for the front cylinder at TDC and then again 45 degrees later when the rear cylinder needs lit.
I installed a single fire ignition on a sporty sport 4 plug engine and they run very well like this. I believe it was a Crane or Accell brand. This setup only charges the appropriate cylinder thus saving the coil voltage wasted on zapping the wrong hole. It also avoids backfiring through the intake because it isnt charging a fresh fuel/air mixture on the rear cylinder by being zapped 45 degress early.
Can I come out of my hole yet?

 

Well you are very close to covering the hole. Excellent summation. I understood all of it except the part about coil being charged twice at a time. This statement is somewhat ambiguous it's like flushing the same toilet twice at one time ? But further when you say the front cylinder gets a coil pulse at TDC and then the rear cylinder gets one 45 degrees later, that really does not come across where I can understand at all ?
Because unless I have misunderstood the whole firing sequence there is one fire (coil pulse) and then 305 degrees rotation later, another fire (coil pulse) and then 405 degrees rotation later another fire (coil pulse) and the repetition continues. There is no fire again at 45 degrees rotation ? What am I missing ?

 

Here's a website that explains the whole idea of waste spark and HARLEY timing!

http://www.nrhsperformance.com/tech_singlefire.shtml

 

Every time either of the cylinders are at TDC compression (minus advance of course for fuel burn time) both plugs are zapped. The single fire adapter for a tach only snags one of the pulses and leaves the extra pulse out of the equation. Have you ever watched a Harley engine with those neon lighted wires run? You can see how the plugs are being double zapped when its running. That may help you understand more clearly.

 

Thanks for all your effort, it helped. And now I think I get the fact that even though the pulse for both cylinders occur at the same point in time, the tach (without an adapter) would pick up two pulses off the coil (going to each plug). Confusing but I can go with that.

And also I did go and read the link given by FBRR and it turned out to be exactly what I was fishing for. I was 10 degrees off in my firing advance assumption, but it also answered all my other curiosities too. After all the googles I had done somehow I missed this gem. THANKS FBRR !
The report was bit long but I will paste just the pertinant part here:
---begin,
Okay, so what does all that mean for the wasted spark? Well, let’s assume 35 degrees of spark advance and look at where the opposite piston is positioned when the fire is lit. If we hit the rear cylinder 35 degrees before TDC, the front cylinder is 80 degrees before TDC on his exhaust stroke (35 plus 45), or just past the mid point. Is there anything in the front cylinder to burn? And if there’s anything to burn, will it generate any pressure on the piston, pushing it the wrong way?

The answer to these questions is “it depends”. First off, how complete was the combustion that occurred 315 degrees ago? Spark intensity, air/fuel ratio, compression, turbulence, and combustion chamber flame propagation will all play a role. The better that burn was, the less potential exists for the wasted spark to do anything. Second, assuming there is something left to burn, how much downward pressure can you generate with your exhaust valve hanging wide open and exhaust rushing out, which is exactly what’s happening at that point in the cycle? Assuming your exhaust system works, of course.

Now let’s look at the other wasted spark. Again assuming 35 degrees of advance, the rear piston will be positioned at 10 degrees after TDC on it’s intake stroke (45 minus 35), or in the very early stages. Is there anything in the rear cylinder to burn? And if it burns, does it cause a problem?

Once again, the answer is “it depends”. The first 10 degrees after TDC of crankshaft rotation hardly moves the piston at all, so the piston hasn’t really had time to start yanking on the intake charge. A charge that, by the way, got yanked in the opposite direction very recently by the front cylinder and isn’t necessarily anxious to get yanked back this way again.

But don’t forget the effect of overlap! Even though the piston hasn’t significantly yanked on the intake charge, the exhaust system might have done it. So, thinking about it logically, it would seem that an effective overlap event, which is driven by cam timing, the exhaust system, and the independence of the carbs, will increase the chances that there’s fuel available to burn when the wasted spark happens in the rear cylinder.

Now, if there is fuel available to burn, and the spark plug manages to light it, it could certainly be disruptive to the incoming charge, resulting in less cylinder fill. It won’t push the piston in the wrong direction, however, as the piston is on it’s way down at this point anyway.

So the answer is a great big maybe.
----end report
Also I found an interesting tid bit at another site that said the compressed cylinder offered less electrical resistance and therefore pulled much of the electrical current away from the other non compressed (intake and exhaust) stroke cylinder on a dual fire. Found no data that Mr. Harley and Mr. Davidson were aware of this electrical advantage when they designed it, but if they were I would be impressed.
Actually in closing, the Orlando (Florida) Harley dealership that I bought my used Street Bob from (right off I-4) is owned by a Gentleman name Steve Deli who's wife actually grew up neighbors with one of the Harley kids. Said she used to ride on the back of one of their Harleys with them. Just thought I would throw that in there. If anyone lives in the Central Florida area and you want to choose from a huge selection of Harleys (new and used) go on their internet site http://orlandoharley.com. There is an excellent salesman there name Chad Martel that will help you find a bike if you email them. (Orlando has 3 dealerships)

 

Before the days of electronic ignition systems, Harleys like my shovel had a single set of points and a single ignition coil. The timing cam that operated the points opened them twice per revolution, which at half engine speed was once per crank revolution. The coil fired both plugs at the same time. It has nothing to do with exhaust emissions, as it predates them! The ignition timing was always done off the same cylinder, simply to provide a uniform way of setting it. The timing of the live spark was roughly the same on both cylinders, taking into account accuracy of manufacture.

You chaps over there have always confused me with your talk of 'single' and 'dual' fire. The old way fired twice on each cylinder, one spark being wasted, the new way fires only once.

Does that help?!

 

I've pretty much got solved now, but thanks for the reply since your older scenario (from what I've read) is exactly why the dual fire was initiated. It was cheaper than putting a distributor to fire each cylinder the way a car does. The whole thing that got me was how they got away with a fire on the intake stroke, but I see all the facets of it now.
Now days with electronic ignition there really would be no reason to have a dual fire.

 

I haven't seen one of those for awhile. Went to the vw aircooled engine and transmission factory school in 71 I are a fact-tory trained meck- aa- nic