Britbike forum Forums British motorcycles in general British motorcycles in general Why waste a spark?

While setting the points on my Triumph a question occurred to me. Why does the points cam have two lobes creating a "wasted spark"?
I have a feeling the answer is going to lead to a forehead slap for me.

None of the dwell cams I've looked at on BSA or Triumph twins have two lobes.

The "lobe" as it were is the high spot after the low flat spot where the coil "charges". After the points open, they may or may not stay open the rest of the way around the cam, but this will not produce a spark until they close once again at that "flat spot" and then open.

I think
What looks like the second lobe
is really just the end of the first Lobes open time .
Before the cam ramps down and the points close
To again
Dwell the coil .
.
For reference, the Google says :
The 4ca has 86 degrees of dwell ( closed degrees )
The 6ca has 160 degrees of dwell .

Points systems on Triumphs and practically all other British bikes were not wasted spark.

Thanks, all. I see it now. That reference would be crankshaft degrees, yes?

, 160° is points cam degrees
Its a long dwell .
I dont have a points cam handy for reference .

Only this great pic .

Notice how the timing side points are charging
and the Drive side points are past firing ...but still open

dwell refers to the point cam itself .,not the crank .
And the number of degrees the points are closed and charging the coils.

This speaks to why the cam looks like it has two firing lobes ... but it doesn't .
It's really a long 160° degrees of flattish arc , dwelling the coils , a high spot that brakes contact ... and a long slow ramp down .

The second lobe-like area , opposite the firing mark ... is the transition
where the points ( finish there open time ) and go back into dwell .

Thank you. That makes it crystal clear.

Now that we've cleared that up, I'd like to take this another, related direction.
My frame has a nice built in mount for this type of coil:

Can I use a dual coil like this with points? Assuming proper impedence.
I've searched several posts here and elsewhere and still am not convinced. Usually the poster replies confuse the question (or I'm confused) or take it on a tangent but some claim to have done it.
The only way I can see it working is if you somehow isolated the points plate from ground and then wired the points in series - and then you would have a wasted spark.
One thing I have not been able to find is a wiring diagram of a setup that works.

Or how about a coil like this?

Any better?
Not much information available about these but I believe they are two coils in one package. Might work.

Well, the bottom pictured coil makes a spark, but each side measures 3.3 Ohms, compared to the 2 Ohms of my Lucas coils. Yet to see if it will run the bike as it's not fully assembled.

12 volts ?

3.3 ohms from center to either side ?

If yes ... the 3.3 ohm , single fire dual-coil should be fine .

For comparison , a standard Lucas 12 volt coil
ohms pretty close to 3.7 ohms .



Your old 2 ohm coils are set up for 6 volts ... or run with a balast for 12 volt ... or would work in series with a 12 volt wasted spark .

Running 2 ohms coils ... at 12 volts .... with the long dwell of the 6Ca points ... does not sound like a recipe for success or long life .

.

Thank you!

How can I tell if it's a 12 volt coil? Yes, the 3.3 Ohms is from the center to either side.

I guess one can tell the volts of a Lucas style coil by the Ohms.

How would I wire the 2 Ohm Lucas style coils in series (If I were using them) on a 12 volt, 2 cylinder, point ignition (the wasted spark)?

Thank you for your patience.

I'm speaking theoretically, but I did hear that someone used a single, dual-lead 12V coil with breaker points and it worked. If this is true, then two 6-volt coils in series would work as well (although I don't know why you would want to do this). This would require that both sets of points would be wired to the ground (+?) side of the second coil. This also implies that the "other" set of points remains open around the back side of the dwell cam, so as not to interfere with the set of points that's closing and opening and producing the spark.

I can’t see how it could work, only other way would be to time the ignition off the crank.

So called 12v coils are better referred to as 4 ohm coils. So called 6v coils are better referred to as 2 ohm coils.

I’ve also seen (own) “Lucas power” coils, they don’t exist now but those are rated at 3 ohms and designed to work within a 12v circuit.

You got it .
But remember
The 6 or 12 volt designation is just a convenience ... matching the ohms to the application is whats more important.
What alan said about 2 and 4 ohms is a good generalization.
2 ohms = 6 volts ... but there are plenty of 12-volt applications using 2 ohm/6volt coils .

A Lucas coil should have a stamp on the bottom .
( well most of them ... and maybe even a clear enough stamp to be read )

Check the stamp against
Primary resistance numbers 
Heres a list i found
In a Lucas work shop manual ... or there is no stamp ...just use the ohms value against reference .
( your 3.3 coil look perfect... the 3.7 number i posted before was from memory)

LA6 ___ 1.0 - 1.1
LA12 ___ 3.0 - 3.4

MA6 ___ 1.8 - 2.1
MA12 ___ 3.0 - 3.4

17M6 ___ 1.7 - 1.9
17M12 ___ 3.3 - 3.8




I wouldnt bother trying to wire dual points with coils in series. .. its a bit convoluted .
The wasted spark is a ballast load for the desired spark .
Its simple enough to
Wire each " 6 volt " coil normally ... with a separate ballast load .
add a 1.5 ohm ballast resistor in series with each coil feed for each set of points .
... total cost for 2 resistors should be from 4 to 10 bucks .

What wattage rating should the resister have?
Thanks

Power = Voltage * Current or Current^2 * Resistance. Multiply by 1.5 - 2 for safety factor so 40W - 50W.

Adding ballast to a coil .

... revision... dont kow why a said 2 ballast resistors ... its easier to visualize as each coil having its own resistor .
But
Only one ballast resistor is needed .
Kill-ignition power is feed in one side .
And the output is paralleled to the 2 separate coils ...
The coils both draw through the same resistor ... during there respective dwells .

Bike is ready to break in so hooked up this coil again and I'm not sure what it's doing but it makes my battery voltage drop and lights don't work. I guess it's shorting somehow(?)
12 volt negative ground system. I wired the center post to the battery positive and the two outer to the points.

I have some Lucas 12 volt coils so I'll try them but I really wanted to use this style coil as my bike has a special mount for it.

Is positive feed going through a key-switch... and/or kill ... on its way to the coil .

Is the bike 6 or 12 volt electrics? If your bike had 2 x 2 ohm coils i assume it's 6v electrics?
Is the bike pos or neg ground/earth?


The wasted spark arrangement has been around for years and was done as a cost saver with both points and EI.
Some people say that it reduced emissions, possibly so. Others say it helps keep plugs clean.
Fact of the matter is, it saves a coil and a set of points or transistor and with a 360 deg twin it's cheaper.
The arc on the wasted side forms a very low resistance when struck so effectively completes the circuit for
the loaded side, this all happens very quickly and is negligible in most cases. The two spark polarities are opposite
so one plug will erode slightly faster than the other, once again negligible. The direction of electron flow is also
slightly better on one direction but we won't go there. Suffice it to say, a double ended coil works well.


To get a standard 2 primary terminal dual/twin spark coil to work with points is a bit fiddly, but it can be done. Personally i wouldn't bother.

If it's a 12v setup.......
With your 3 terminal 3 ohm per primary coil..........
My first suggestion would be to remove the condensers/capacitors from the points plate and fit/wire them up by the coil.
A single 1 ohm 25watt resistor can be placed in series with the centre feed terminal as your ballast if required.
Each set of points would be supplied by either outer terminal.
The snag here is that one side of the primary will always be on so dependant on how the coil is wound, it's effective
spark may be reduced. Some are designed/wound to work with both primaries being open at either spark. (Some EI setups.)


Fitting an EI makes life so much easier............................................Think about it.

My 2c of course, only my take.

Again, it's 12V negative ground. Not trying to waste a spark here.

I'm not losing voltage at the key switch. And the condensers are mounted by the coil to the motor.

I could try the 1 ohm resistor but I can't say I understand what that does. Or was that only for if I was using 6 volt coils?

I'll look again today.

I did a lot of searching of the web and can't find a simple wiring diagram using this type of coil with points - if only to confirm what I'm doing.

Well, it works. Which is quite exciting because today was the first time it's run since a total tear down over the winter.
Mikuni carbs need some adjustment, also timing and charging system need to be checked before I give it a break in run.

The voltage will drop when the ignition is turned on , remember the long dwell time of this ignition cam .
... there is only 40° or 11% of the time were the coils are off . .. and this is when the bike is running.

1. Coils good... battery not so good . ( its quite common to over estimate the batteries capacity )
.... the good news is the ignition doesn't really need much of a battery ... if the alternator is working

the coils will have a 3.3 ohms inrush load ... for most spots... the ignition-cam randomly ends up
... when the ignition is on
and the bike is not running....
left on ...in this condition... the coils are little electromagnetic heaters ... that are getting "over dwelled" .
This is normal for points , but the quality of the coils will ultimately determine how much normal-abuse they can handle .

If your battery is " half-bad" ... the ignition load itself ...can pull the voltage down quickly .

2. battery good ...Coils bad .
... or the coils themselves could be cooked ... are the coils new or something from a box of parts .


3. Other wiring faults ... not related to ignition... are responsible for quick voltage drop .

A. Are we taking about stock wiring .
With the one change being ... this dual single fire coil ?

Exactly the reason why I fit an idiot light in the spot where the oil pressure switch light would be on my headlight bucket (no OP switch on my 68) I kept leaving the ignition on, come to it the next morning with a warm coil and a dead battery. Praise the 3 phase ignition, it still started 1st kick when on points with a dead battery.

NickH Again, it's 12V negative ground. Not trying to waste a spark here.

With that coil you will will get a wasted spark. Either set of points will fire both towers.


Allen. Exactly the reason why I fit an idiot light in the spot where the oil pressure switch light would be on my headlight bucket (no OP switch on my 68) I kept leaving the ignition on, come to it the next morning with a warm coil and a dead battery. Praise the 3 phase ignition, it still started 1st kick when on points with a dead battery.

With 90% of EI's the coil will be switched off after a few seconds of no trigger signals. (stall timer) The battery drain
/power consumption on a typical EI is around 50ma. when not running.