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Why waste a spark? #768139
03/14/19 11:00 pm
03/14/19 11:00 pm
Joined: Oct 2017
Posts: 169
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Nick H Online content OP
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Nick H  Online Content OP
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Joined: Oct 2017
Posts: 169
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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.


1966 BSA Lightning
1967 Triumph "Choppa"
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Re: Why waste a spark? [Re: Nick H] #768141
03/14/19 11:14 pm
03/14/19 11:14 pm
Joined: Aug 2001
Posts: 4,692
Owego, NY, USA
Mark Z Offline
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Mark Z  Offline
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Joined: Aug 2001
Posts: 4,692
Owego, NY, USA
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.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Why waste a spark? [Re: Mark Z] #768144
03/14/19 11:27 pm
03/14/19 11:27 pm
Joined: Sep 2013
Posts: 1,916
Pacific northwest
Q
quinten Offline
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quinten  Offline
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Q

Joined: Sep 2013
Posts: 1,916
Pacific northwest

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 .




Last edited by quinten; 03/15/19 12:17 am.
Re: Why waste a spark? [Re: Nick H] #768165
03/15/19 6:32 am
03/15/19 6:32 am
Joined: Aug 2001
Posts: 7,779
scotland
triton thrasher Online content
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triton thrasher  Online Content
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Posts: 7,779
scotland
Points systems on Triumphs and practically all other British bikes were not wasted spark.


Amateur Loctite enthusiast.
Re: Why waste a spark? [Re: quinten] #768208
03/15/19 9:36 pm
03/15/19 9:36 pm
Joined: Oct 2017
Posts: 169
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Nick H Online content OP
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Nick H  Online Content OP
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Joined: Oct 2017
Posts: 169
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Originally Posted by quinten

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 .




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


1966 BSA Lightning
1967 Triumph "Choppa"
Re: Why waste a spark? [Re: Nick H] #768224
03/15/19 11:20 pm
03/15/19 11:20 pm
Joined: Sep 2013
Posts: 1,916
Pacific northwest
Q
quinten Offline
BritBike Forum member
quinten  Offline
BritBike Forum member
Q

Joined: Sep 2013
Posts: 1,916
Pacific northwest

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

Only this great pic . [Linked Image]

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 .

Re: Why waste a spark? [Re: Nick H] #768278
03/16/19 5:54 pm
03/16/19 5:54 pm
Joined: Oct 2017
Posts: 169
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Nick H Online content OP
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Nick H  Online Content OP
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Joined: Oct 2017
Posts: 169
MA
Thank you. That makes it crystal clear.


1966 BSA Lightning
1967 Triumph "Choppa"
Re: Why waste a spark? [Re: Nick H] #768382
03/17/19 11:14 pm
03/17/19 11:14 pm
Joined: Oct 2017
Posts: 169
MA
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Nick H Online content OP
BritBike Forum member
Nick H  Online Content OP
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Joined: Oct 2017
Posts: 169
MA
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:
[Linked Image]
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?
[Linked Image]
Any better?
Not much information available about these but I believe they are two coils in one package. Might work.

Last edited by Nick H; 03/18/19 4:25 pm.

1966 BSA Lightning
1967 Triumph "Choppa"

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