The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: bsa-bill on 31.10. 2010 19:46

Title: Is this a silly idea
Post by: bsa-bill on 31.10. 2010 19:46
Just been looking at an article about systems that produce a spark every revolution of the engine, the spark on the exhaust stroke being redundant or wasted as the term is.
I had a thought that this could be a cure for the uneven timing sometimes experienced due to uneven cam rings.

Now tell me would our magnetos produce enough power to give a spark to both cylinders at the same time, I'm thinking if both pickups were linked together and with two plug leads coming from the one link any difference in timing of the sparks would be negated, might even be a plus.

OK why would it not work, I don't mind sticking my head above the parapet just don't throw bricks
Title: Re: Is this a silly idea
Post by: wilko on 31.10. 2010 20:03
I wrestled with that idea once but was too embarrased to ask. Watch out for the bricks Bill!
Title: Re: Is this a silly idea
Post by: a10gf on 31.10. 2010 21:07
Fascinating thought.

Am looking forward to others views on the question. But initially it looks like there may be some viable reasons for why nobody has done this during all the years of twin magneto existence!

edit 1: removed some funny, but wrong, ideas (hopefully before it got indexed by google!)
edit 2: I would believe there could be no guarantee that both plugs would give a spark, resulting in either the plug in best condition sparking, or some random results.
edit 3: but.. a mag, single or twin (using one output), if it was made to revolve at twice it's normal speed should work (timing problems gone).... no?  *smile* (and magneto probably self-destructing).
Title: Re: Is this a silly idea
Post by: MG on 31.10. 2010 21:52
Basically a good thought Bill, BUT:

First of all, like E mentioned, a Lucas mag rotating at crank speed (6000 rpm+) will disintegrate, for sure, no doubt.  *smile*

Secondly, just think of the problems you sometimes encounter. Have you ever had a bad spark plug that seemed to work perfectly alright when outside of the engine, but as soon as you put it back in the head it would refuse to ignite the fuel-air mixture? Or have you ever had a soft pickup brush leaving a conductive carbon track around the slipring, resulting in misfiring?
These are effects of the compressed air-fuel mixture in the combustion chamber. When exposed to the high pressure, it will need much more energy for the spark to jump over the elctrode gap.
Hence, if you connect the two pickups together, even if the electrodes and their gaps were absolutely identical (which can't be achieved anyway), the spark will almost always appear on the cylinder that is on exhaust stroke. The same happens with the aforementioned carbon track, the spark will "run" along the carbon to the other pickup and to the cylinder on exhaust sroke, rather than going straight through the brass insert and to the correct pickup.

Cheers, Markus
Title: Re: Is this a silly idea
Post by: bsa-bill on 31.10. 2010 22:06
Markus and A10gf - I was not thinking of speeding the magneto up to twice the speed, but using the existing setup and joining both pick-ups together so getting two sparks per magneto revolution but each spark would go to both cylinders simultaneously.

Quote
Have you ever had a bad spark plug that seemed to work perfectly alright when outside of the engine
Oh yes that has been the bugbear of my RGF project, so yes that's a good point

Ah well maybe Joseph probably knew what he was doing
Title: Re: Is this a silly idea
Post by: MG on 31.10. 2010 22:09
Quote
Ah well maybe Joseph probably knew what he was doing

Err, yes, but having the choice between a simple, robust design and a complicated, fault-prone solution for a given task, guess which one he tended to use.  *whistle*

Title: Re: Is this a silly idea
Post by: chaterlea25 on 31.10. 2010 23:42
Hi Bill,
Basically there is not energy produced by the mag to spark 2 plugs at the same time!!!
A couple of years ago the slipring on my SR shorted around the "plastic" bit and the bike was trying to fire the 2 plugs at the same time *ex* *ex* *ex*
Result, roadside rescue, If I thought of it I would have closed up the plug gap real tight and I may have gotten home ???? ????
Funily enough the oposite works fine!! on my Ariel single which has a BTH (old type) mag
When I built it I didnt have cam for the mag, an old boygave me a twin oneand said it will be fine *ex* *ex* *ex*
Well its been in there providing twice the amount of sparks since 1984 *smile* *smile*

Cheers
John O R
Title: Re: Is this a silly idea
Post by: bsa-bill on 01.11. 2010 10:05
Hi John - was no sparks at all the result of it trying to fire two spark at once via carbon track around the slip ring, I ask because this is what has happened to the Magneto from my Flash (that has run faultlessly for ten years).
I swapped it onto the RGF and it started fine and ran fine until I stopped to talk to a friend, then it would not start - no spark, changed the plugs - still no spark, I was thinking warm starting problem but it was reconed ten years ago.
I have it off at the moment and intended to give the condenser and windings a check ( someone did tell me what resistance to look for across the windings with the condenser de-soldered - was it you).
Or I might give the slip-ring a good clean and try it again in light of your experience , OTOH it did start after it had cooled so might pay to check it out before refitting
Title: Re: Is this a silly idea
Post by: olev on 01.11. 2010 11:06
Gday Bill,
I reckon Marcus is right.
You will have trouble firing both plugs equally.
The electronic ignitions that use a wasted spark have the plugs wired in series.
The high voltage winding of the coil is a closed loop and not earthed.
The current path (when the field collapses) is through one plug, across the metal head, through the other plug and back to the coil.
So its either both plugs fire or none.

On a tangent, if a motor had electronic ignition and was turned over to just past TDC and the plugs then fired using a push button, would it start?
Title: Re: Is this a silly idea
Post by: chaterlea25 on 01.11. 2010 13:38
HI Bill, an All
Bill, I have since found several mags where the slipring was conducting across the plastic part, low resistance even being read by a digital meter *ex*
When my SR gave bother, it would run on one cylinder fine with the other plug lead off, and the same on the other cylinder, It missed and farted etc with boh leads on *eek*
The slipring fault is the same hot or cold

Olev,
Tangental answer *smile*
Yes, sometimes!
with electronic or battery and coil ign the system sparks when the supply to it is broken/made??
I have seen car and bike engines turn when the ignition was turned on/off
Vintage cars with manual ignition control (Ford A's) will sometimes start when you go from full adv to retard and back on the lever *ex* *ex*

Cheers
John O R