Topic: Ignition question-4 plugs, 2 coils

[jgary]

I recently replaced points, condensers, plugs, and wire end caps on a CB750 that my neighbor gave me.  Got the bike running, but I've been troubled by a question since then.  One set of points fires plugs 1&4, the other set fires 2&3.  Each coil has two plug wires coming out, so how does only one plug fire at a time, or do two plugs fire at a time?  This is causing confusion and delay.  Thanks!

John.

[apr1967]

Two plugs fire at the same time.

Often called a wasted spark ignition.

[Bob Wessner]

It avoided the need for the complexities of a true distributer. The one plug fires on compression, the other, "wasted spark" fires on exhaust.

[Gordon]

That one confused me too the first time I worked on my 400F.  Another that got me was trying to set the points gap with a dwell meter.  I was having a very dificult time until I realized I had to put a piece of cardboard between one set of points to keep them from firing so I could get an accurate reading on the other set.  Once I figured it out, it was comforting to know that I could lose half the ignition system and still limp home on two cylinders.

[Mark M]

Wasted spark is standard pactice on 4 cylinder bikes.

[kaysystems]

Some of the electronic ignitions fire twice per rev, creating 3 wasted sparks. Surely one at the bottom of the intake stroke messes things up?

David   

[Bob Wessner]

I know the mult. firings of elec. ignitions can't literally happen at the same time, but I always thought they occurred in "very" rapid succession to insure all fuel had been ignited??

[kaysystems]

My Boyer Bransden has two coils 180degrees apart & two magnets also 180 degrees apart, which means that a magnet passes each coil twice per engine revolution.

David   

[Quail "Owner of the comfortable k8"]

The field is only created when the points are closed, so when the coil passes over the 180 degree the points are open no field is created.  The cam shaft only turns 1/2 the rpms of the crank. 

[the_genrl]

pretty much explained but anyway.

  pistons 1-4 and 2-3 are in the same position, but for each matching position, say 2-3, the cam is 360 out of phase from each other (or 180 if youre lookin at the cam).  honda, being cheap, must have said "hey 360 is 360" and put only 2 spark units on the CRANK.  this way it'll fire a: when it needs to AND b: right between exhaust closed and intake open (useless).  think its called a "hit or miss ignition."       remember, if the points were on the cam, theyd need 4 points.

hey it works...look at anything that has its ignition generation on the crank and it uses the same deal (cept modern car crank position sensors) 

now can you tell me how they get away with 6 lobes, 2 points, and 4 cylinders?  *hint: calc the deg between the points *

[cben750f0]

my head hurts...

[kaysystems]

The field is only created when the points are closed, so when the coil passes over the 180 degree the points are open no field is created.  The cam shaft only turns 1/2 the rpms of the crank. 

But the ignittion is on the crank (1970 CB750)

David

[Jonesy]

I think it was Mallory that offered an aftermarket distributor ignition for the 750. This way, all the juice goes to the plug being fired, making for a stronger spark. You also didn't get the funny wear patterns in the plugs, with the center electrode on one being worn while the corresponding plug the other plug wire was attached to (say plug #3 from #2) would wear off the ground electrode, since electricity flows in a loop. So, the current would flow "down" one plug and "up" the other. There was a website that explained all this better, but I need to hunt it down...

[dusterdude]

harley-davidson used the wasted spark technique forever it seems like.