The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: Terryb on 28.03. 2020 17:29
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Can anyone tell me why, when checking the timing using a strobe light. It works fine on one cylinder, but not the other? I've tried swapping the leads over, but still only works on the one lead. The bike is 1955 A7SS fitted with electronic ignition in the mag. Thanks
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IIRC if your using a strobe light that clips on a plug lead it matters which way the clip is on the lead, or put another way which way the charge that produces the spark travels through the clip.
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Daft as it seems, is it actually running on two cylinders?
Single feed in, twin output lead coil, dead spark system? You would think the output from each plug lead would be the same. Maybe it is designed that way, and no one else has bothered checking both leads with a strobe. Is the spark the same from each lead, or one a definite weakling?
Swarfy.
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Should work. Maybe try putting the clamp the other way around on the lead that does not trigger, as my findings shows one spark voltage is negative, the other positive. Your strobe may mind :O)
https://www.a7a10.net/forum/index.php?topic=1375.msg9915#msg9915
Spark pictures:
https://www.a7a10.net/forum/index.php?topic=1412
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To be honest, I've not tried reversing it, I'll have a go in the morning. Thanks
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Well do it
Double ended coils produce a + spark at one plug & a - spark at the other
The inductive pick up generally has a arrow on it so upside down should work fine.
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Ok.
I've checked the spark strength visually on both leads and they appear to be both the same.
I've reversed the electrode on the No.1 lead and nothing.
I've checked the No.1 without the No.2 plug lead connected, nothing.
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Was replying too fast, now reading it's an electronic, then there should be no + or - spark (?).
Am curious, post a picture of your test setup\device ?
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Don't have a Test Rig, but here is the Thorspark trigger unit and Twin Coils.
Took the bike out for 5 mile run and seems to run OK, pulls well, no pinking and ticks over nicely. Still getting kick-back when starting, so I'm going to to have another look at the timing, it's such a tiny adjustment to the rota which makes a big difference to the timing.
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He's a daft question.
What if I set the timing up on No.2 Cylinder?
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With no strobe output from that lead, how would you do it?
Time for a call to Thorspark, or perhaps someone with the same system can give it a try. I thought Thorspark used a single coil with twin outputs sparking both cylinders at the same time, a so called dead spark arrangement. So maybe I'm wrong.
If you have sparks from both, it runs OK, leave it alone for now and put it down as a mystery.
RoyC fitted a Thorspark, worth an ask.
Swarfy.
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RoyC, thanks for the input. I will call Thorspark. I think your right, I just need someone to assure me that if I swap the leads over the spark that I can see from the Strobe is for that cylinder. In the past I've had no problem using the strobe on either lead, somethink has changed!
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Thorspark has said that you can swap the leads over. *smile*
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A question from the electrically-challenged :-
I have an old Gee-Jay Timeoscope Model 33 which I purchased in about 1969. I used it on the old 6 Volt Holden FX, but the strobe light was a dull yellow - a close match for my aging eyes. *cry*
Time to get a new one. I haven't used a strobe yet, but keen to give it a go. What recommendations for my magneto A10? Preferably cheap - ALDI, Supercheap etc.
Col
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G'day Col.
This is the Super Cheap one I use https://www.supercheapauto.com.au/p/toolpro-toolpro-professional-angled-timing-light/385507.html
There is one similar from Onlineautoparts https://tinyurl.com/3eu86frc
They tell you the revs while your strobing.
I use an external 12v battery rather than the bikes.
Cheers
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Ta Musky, I've ordered one.
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The new timing light has a separate lead and clip for measuring Dwell. I haven't measured Dwell since I used it on my 1952 Holden 35 years ago. I don't think I have ever thought about Dwell on our magneto bikes. Although it would be nice to know what the dwell is or should be, but is it possible to measure it on a magneto? If so should it take priority over points gap? and where would I connect the Dwell clip? I am sure Bill Groily or Andrew Cheeserbeeser will have an opinion on this??
Col
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Not much of an opinion as such from here Col . . .
. . . But, the practice on twin cyl camring mags was typically a ratio of Points Open to Closed period of 0.8 - so on a K2F it would mean points closed for 100° of rotation x 2, open for 80° x 2. On a 4 spark per rev mag those figures would be halved. On a single, much more points open time - as the coil charges up fully in the 100° points closed sort of arc of movement.
Camrings should offer the right ratio if the points gaps are set correctly, and while it might be recommended to focus on the dwell more than the gap on some distributors, not I think a good plan with mags as a big gap causes wear on the contact breaker and a small one risks arcing.
A small points gap will increase the dwell a bit, and a large one reduce it, as the points open earlier with big gaps. Wear will also be a factor probably.
I haven't ever tried to measure the effect of reduced or increased dwell on HT voltage with a mag. Maybe someone has?
The two priorities are that the gaps should be reasonably close to equal (within a couple of thou), and above all that the firing interval is as close as you can get it.
I have only ever measured the ratio (just using a rotary table) as an incidental activity when faced with worn camrings that need tweaking to get even firing. I haven't noticed a performance penalty as a consequence of grinding the cam lobes a tad. You could probably measure your dwell with a degree disc on the spindle and something to tell you when the points open . . . If you wanted!
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Hi Bill, That all makes sense. I'll leave the Dwell clip off then.
Col