Degrees between fully advance and fully retard

[Yves.Astein]

Hi fellow cyclists,

can anyone provide the amount of degrees between fully advance and fully retard setting for a 51 G3 ? I guess it has the N1-4. I am using a degree disc to set the timing at the crankshaft. According to the book I need 39 degrees fully advanced. If I measure the difference between fully advanced and fully retard I get a difference of over 40 degrees. So at fully retard I end up with a couple of degrees after TDC. That can't be right ! The screw limiting the travel of the cam disc is there and seems to be doing its job.

Any help appreciated.

Yves

[Groily]

Bonjour Yves,
No, it is right. Generally, there are at least 20° of movement on the camplate of an N1 or an MO1 magdynamo. Far more than is needed.
Which is why I always say to folk - 'half retard should be plenty for starting'.
I have seen 23, even 25° of movement on some. But it doesn't matter (as long as the plunger doesn't disengage due to excess travel) - what matters is being sure to set the timing with the magneto fully advanced.

For what it's worth as a comparison, the average twin cylinder manual cam ring will give about 18° of movement, 36° on the crank. Also more than is needed..

For what it's also worth, no auto advance unit offers more than 18° (and those are for Tr****s). Most are in the 11-13° range, which provides a perfectly adequate amount of retardation.

And it is very relevant that the standard Auto units on AJS/M machines with SR1 magnetos, Lucas parts 47541 (500s) and 47544 (350s), provide 11-13° of movement. Which says that 'half retard' on the manual N1 is indeed quite enough!

It is also worth just bearing in mind that with the manual mags, the more retard the weaker the spark. Which is unhelpful when one thinks a magneto is at its weakest at kickstart speed, and we then make it weaker still by upsetting the internal timing on purpose to save our legs! For this reason, I like atd units, primitive though they be.

[Yves.Astein]

Thanks Groily,

for your detailed answer !
Now I understand.
I guess to much retard will not hurt anything. I was so focused on doing it right, that I forgot the pragmatism the people had at that time, when designing our bikes.
I had the same problem my G12, trying to limit the rotation of the cam ring to 24degrees. I ended up limiting the travel with the leaver as one stopping point in one direction and the stopping screw for the cam in the other direction.
But you are right, as long as the advance setting is limited to the right amount, who cares about the rest, except for the spark intensity of course.

Cheers Yves

[Andy G]

I've read a number of times that spark is weaker when mag is in retard, but I've never understood why?
Surely the mechanism is rotary and symmetrical, the points separation is effected by the cam ring at whatever position it's in, everything else stays the same...

Or are folks referring to the mechanics of the resulting combustion after ignition, slower propagation etc...

I'd love to understand this better, can anyone enlighten?

[SPRIDDLER]

Here you go, Andy.........

https://old-engine.com/maglma.htm

[Yves.Astein]

I've read a number of times that spark is weaker when mag is in retard, but I've never understood why?
Surely the mechanism is rotary and symmetrical, the points separation is effected by the cam ring at whatever position it's in, everything else stays the same...

Or are folks referring to the mechanics of the resulting combustion after ignition, slower propagation etc...

I'd love to understand this better, can anyone enlighten?

Hi Andy, its quite easy. The simplified explantation is:
The magnetic field that inducts (inserts) energy into the windings of the turning armature is not evenly spread within the housing. Its even so, that between the two magnetic field directions (north and South) there is a zero point. In addition the armature is also wound unevenly.
So on a 360 degree turn, the fieldstrength is distributed comparable to a sinus curve. Having a peak and a zero crossing.

Hope its understandable...

Cheers Yves

[Groily]

As Yves says.
The further after the point of flux change of the armature that you ask the points to open, the weaker the spark.
Too early (before the 'flip') and the HT coil won't be charged up; too late and you're wasting a fair bit of the oomph.

On a bench test, it is easy to see the very rapid deterioration in low speed performance of any magneto if the points open later than they should.
If, say, a decent N1 will make all its sparks to Lucas specs at somewhere between 110 and 130rpm cold at full advance, it will need around another 30rpm when retarded, and if the camplate isn't set accurately or the points unit is badly positioned on the armature owing to the typical wear on the jaw of the spindle, it will need more in all positions. Doesn't sound all that significant - but the crank has to be turned at twice those speeds to start the beast. And when hot, they always need more revs again . . . another 40-odd rpm at the mag, 80 at the crank, wouldn't be unusual.

The reason Auto Timing Units are 'good' (despite being very poorly designed in many respects) is that the magneto itself is always firing at its peak moment - the drive train is what changes, not the internal timing of the mag.
So Vincent, for example, with the inherent problem of any magneto on any V twin, were early adopters of ATDs. Indian, for another example, went to battery and coil ignition early on - before their dynamos were, er, reliable really- for that reason. Pre-war HRDs with manual magnetos, and mags fitted to JAP engines, maybe Model Xs in our world, are asking their magneto to work - when retarded - up to 50° or even more off the optimum, which is a very big ask and they can be very hard to get to perform the way we all want them to on their second cylinder. (The larger the V angle the harder it gets, and so despite my personal attachment to magnetos, I am rather in favour of electronic units like Thorspark's one for V engines!)
Bosch modified their armature designs to have stepped or 'overhanging' sides on their bobbins in an effort to improve matters on Vs, and that helped a bit, but asking something designed specifically to operate at 180/180° to operate as far off kilter as 150/210° is quite a heavy duty demand.

The most inventive design I ever saw for a V engine was on a Moto Rêve, Swiss made, pre WW1. It used an eccentric drive train to let its mag fire at 180/180° despite the V - which confused the heck out of me when overhauling it as I had no idea why it was working symmetrically, not having the engine to look at!

[Andy G]

Well thanks all for the explanation, makes sense now!
My main error was actually in the assumption that there was symmetry in both the Magneto internals and totally ignoring the sine wave effect of the generated current.
I could sleep easier now if my confusion wasn't replaced with a fanatical urge to keep rechecking my points!

Thanks again