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Ignition Advance?

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
have i got this right, if I increase the points gap from 0.012 to 0.014 this will have the same effect as advancing the ignition without the tiresome job of resetting the ATD Will this cause the fully retarded setting also to advance which may make starting a little more difficult?

I'm sure I saw a post on this by Big Sid but can't seem to find it.
 

greg brillus

Well Known and Active Forum Website User
VOC Member
Yes you are correct....smaller gap retards timing and the reverse for a larger gap. This is only useful if you want to change the timing by a couple of degrees either way though. Whatever you change at full advance will do the same at the retarded position via the ATD. And altering the points too far can also upset the internal timing of the magneto itself, that is that if you advance the timing too much the spark can disappear all together. It is not quite so bad in the retarded position. Cheers......Greg.
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
Greg is right on how a timing and mag function is changed by point gap change. A coil ignition also works the same way. The coil is made up of a few turns of heavy copper wire wrapped around many turns of fine copper wire. The heavy winding is the low voltage or battery side of the coil. The amount of time the points are closed determines the saturation of the primary circuit. When the points are opened the collapse of the magnetic field of flux crosses the secondary set of fine wire windings causing a high voltage of around 30,000 to 40,000 V for a very short duration of time. This high voltage is directed to ground at the spark plug electrode causing spark. The points, when opened, have a tendency to draw a spark as all electrical circuits do. A condenser or capacitor is employed to aid in the longevity of the points.
 
Last edited by a moderator:

nkt267

Well Known and Active Forum Website User
VOC Member
And altering the points too far can also upset the internal timing of the magneto itself, that is that if you advance the timing too much the spark can disappear all together.
I had to think about this for a moment....I think this is true when you have a manual advance/retard mechanism as you move the opening point in relation to the mag body.With an ATD in the timing chest surely the opening point of the contacts will always stay in the same position, you only change the relationship of points to crankshaft. Ifd you open the point gap too far then they will never close and then no sparks..john
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
With an ATD in the timing chest surely the opening point of the contacts will always stay in the same position, you only change the relationship of points to crankshaft.
It's easiest to think about the effect of changing the points gap when at either low or high rpm where the ATD is at one limit or other of its travel so it isn't a factor. Imagine things are running fine and the engine, via the gear on the ATD, has rotated the magneto's armature until it is just about to open. That is, until the rubbing block on the points is just about to be pressed upwards by the cam.

Now, in this thought experiment, reach in and adjust the "fixed" point upwards by 0.002". Doing this will reduce the gap once the points are fully opened by the same 0.002". Since the "variable" point is supported by the "fixed" point until it is lifted by the cam, in doing this you also have moved the rubbing block 0.002" away from the cam, although nothing else (e.g. the ATD) has changed at all. Because in decreasing the gap you have pushed the rubbing block away from the cam everything is delayed (retarded) until the engine, via the gear on the ATD, rotates the armature just a bit further than before, until the rubbing block once again makes contact with the cam.

So, what this means is that whether or not you have an ATD, decreasing the points gap retards the ignition. However, especially with a magneto, there is a price to pay for adjusting the timing this way. A magneto works by having the points open to break the primary circuit at the same time the magnetic flux through the armature changes sign because of the shape of the armature and magnetic poles. That is, quite independent of the engine, a magneto generates its maximum output if the points open at a specific position of the armature. If after doing this you change the gap, you will decrease its output.

On a Lucas KVF the slope of the cam is 0.00334"/degree(engine). That is, changing the gap by ~0.003" changes the engine timing by a degree. However, making this change will reduce the secondary voltage at 500 rpm by ~7% so it means if you "adjust" your timing this way you will have to kick ~7% harder to start your Vincent.

It turns out Lucas (and BTH) did us a big favor with the notch that's on the armature shaft and the projection on the points plate. This alignment automatically ensures the points open at the position of maximum magnetic flux reversal if the gap is set at the recommended value. Were it not for this alignment, timing a magneto would be a much more difficult process.

So, Step #1 is to set your magneto for maximum output by adjusting the points gap to the correct value. Step #2 is to set your magneto to fire at the correct position BTDC for a given engine by means of the gear you will lock to the shaft of the armature. I suggest you skip Step #3 -- messing with the gap to adjust for any mistake with Step #2 -- unless you are happy with a reduced output from your magneto.
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
thanks for the input, bikes starting and running well but i have this nagging feeling that if occassionally i don't experience pinking when pulling hard then the timing is some what 'soft'.

out on a run today and went up Ditchling Beacon (England, Sussex) which has a serious gradient,but never experienced pinking once. I know people run scared of this condition but you can always 'ride' around it and it does give reassurance that you are pushing things t the limit.

or am i wrong?
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
Greg is right on how a timing and mag function is changed by point gap change. A coil ignition also works the same way. The coil is made up of a few turns of heavy copper wire wrapped around many turns of fine copper wire. The heavy winding is the low voltage or battery side of the coil. The amount of time the points are closed determines the saturation of the primary circuit. When the points are opened the collapse of the magnetic field of flux crosses the secondary set of fine wire windings causing a high voltage of around 30,000 to 40,000 V for a very short duration of time. This high voltage is directed to ground at the spark plug electrode causing spark. The points, when opened, have a tendency to draw a spark as all electrical circuits do. A condenser or capacitor is employed to aid in the longevity of the points.


Why would a moderator take time to pick out and selective edit a contributor to this forum. Is its purpose to help and correct, or is it to belittle ones scholastic skills, using his post as moderator to do so? If it is to make this forum grammatically and spelling correct, he has his work cut out for him !!! He could start on this thread correcting others, I find the moderators actions not funny, child like, and not gentlemanly .
 

Graham Smith

Forum Website Webmaster
VOC Member
VOC Forum Website Administrator
I find the moderators actions not funny, child like, and not gentlemanly .

For Christs sakes Max, please give it a rest.
Most members of this forum really do welcome your obvious wealth of knowledge on Vincents, but if the price to be paid is to be subjected to this continual nonsense, then I for one can do without it.
Many people (including another who I met today) left jtan because of a similar situation, and have complained to me that this forum is now starting to reflect all that was bad with jtan, and as forum administrator, I'm not prepared to go down the road where users feel they no longer want to visit us because of the obvious bad feeling.
Interestingly, the Moderators of this forum (from many countries) until the last few weeks have never really had the need to actually 'moderate'. That may well speak volumes about what's going on at the moment.
Please do feel free to use the forum sensibly, but please think twice before posting and if in any doubt, give it 24hrs before hitting the 'send' button!
Regards

Graham Smith
 
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Bazlerker

Well Known and Active Forum Website User
VOC Member
Speaking only for myself...I corrected your mis-spelling of Thermocouple in order to help you make your point without looking foolish. I know that Alyn's goal was similar..
 
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