Magneto spark

[Erich]

I have coil and battery, so this is not about Austin 7s. On another forum, selling a Matchless with a magneto, a poster has said that retarding the spark is not necessary and in fact that the magneto produces a weaker spark when retarded. I always retarded the spark on my motorcycles to avoid kick back. But more importantly, I don't see how retarding the spark would weaken it. Shouldn't the mag produce a consistent spark but retarding just causes the spark to occur at a different point in the engine's cycle?

Erich

[orbiter418]

In electrical theory , the faster the magneto turns the fatter the spark will be .

Merv

[Peter Clayton]

I have coil and battery, so this is not about Austin 7s. On another forum, selling a Matchless with a magneto, a poster has said that retarding the spark is not necessary and in fact that the magneto produces a weaker spark when retarded. I always retarded the spark on my motorcycles to avoid kick back. But more importantly, I don't see how retarding the spark would weaken it. Shouldn't the mag produce a consistent spark but retarding just causes the spark to occur at a different point in the engine's cycle?

Erich

You're quite right Erich. The spark should not be affected by ignition advance. My AJS would throw you off if you tried to start without retarding and needed the decompressor  to get past TDC before kicking over.

[Timothy P]

I'm afraid I have to disagree with you Erich and Peter

A properly (Internally) Timed magneto will produce a weaker spark the further the timing is retarded (Via the advance/retard lever).

I have copied the following from THIS site.

As the magneto rotates, there is one definite position where the best efficiency is obtained. This is just as the edges of the armature move away from the pole pieces causing the magnetic field to collapse and it is at this position that the fibre heel needs to have risen up the cam enough to start to open the points. This is known as the internal timing of the magneto and most manufacturers provided a means of fine tuning this position. It would have been adjusted to the optimum position as part of the manufacturing process and would not normally need to be changed unless parts such as the cam ring were replaced. Adjusting the points gap by moving the fixed half of the contact breaker points will cause the points to open when the fibre heel is higher or lower on the rising edge of the cam - closing the gap retards the timing, opening the gap advances it. In short, adjusting the size of the points gap alters the internal timing of the magneto.

If the internal timing is advanced too much (by having a large gap) the points may open before the field has collapsed – resulting in no spark at all. The physics of a magneto's method of operation means that there is some automatic advance as the revs go up. It may be that a magneto with too much advance will spark at cranking/kickover speeds and low revs but this additional automatic advance as the revs increase may be enough to lose the spark altogether at higher revs.

If the internal timing is retarded too much (either by having a reduced gap and/or by operating the advance/retard mechanism) the rate of magnetic field collapse will have slowed down by the time the points start to open so the spark will be less intense. Magneto efficiency increases with speed of rotation so this will be less apparent at higher revs but on a 'tired' magneto starting may be difficult and it is not unusual to have no spark at all at full retard at cranking/kickover speeds. (Watford, Splitdorf and Scintilla, amongst others, found a way to get round this but that's another story).

All of the above references to timing are concerned with the internal timing of the magneto. This is totally different to the ignition timing of the engine which is all about making sure the points open at the correct position of the piston as it approaches the top of the cylinder bore. Yes, the ignition timing could be fine tuned by adjusting the size of the points gap but doing so would sacrifice magneto efficiency and is not recommended.

Will the spark be sufficiently diminished to be a consideration when retarding the ignition to hand crank or kick start? If the mag is working as it should, then I wouldn't expect so.

[Mark R]

I agree, I seem to remember the magnetic flux (if that's the right word) diagrams in magneto books. The highest voltage is available as the armature windings passes through the magnetic field at the optimum angle (90 degrees so it's passing through the magnetic field not along the lines). As such I'd agree that there should be a theoretical optimum, how much this would be however is debatable. I do hope this makes sense? 

I've just done a Google image search and attempted to attach it, partly to see how the new system works but hopefully it shows the principle, in it's current position it's passing along the flux lines, when horizontal it'll be passing through them and as such in the horizontal position is where the voltage will be highest.

  flux3.gif (Size: 4.71 KB / Downloads: 175)

[Robert Leigh]

I have coil and battery, so this is not about Austin 7s. On another forum, selling a Matchless with a magneto, a poster has said that retarding the spark is not necessary and in fact that the magneto produces a weaker spark when retarded. I always retarded the spark on my motorcycles to avoid kick back. But more importantly, I don't see how retarding the spark would weaken it. Shouldn't the mag produce a consistent spark but retarding just causes the spark to occur at a different point in the engine's cycle?

Erich

Retarding the timing of a magneto usually moves the contact breaker backwards in relation to the armature. When the mag is assembled it is normally set so that the points open at full advance at the point of rotation of maximum magnetic flux change in the armature, which gives the strongest spark. when the points are retarded the magnetic flux change is lower when they open, so the spark will be weaker. This is not neccessarily a problem, because when starting there is less compression, so you can get a spark at the plug with lower voltage than when fully advanced. Even a fairly weak spark will start an engine, but it will not give good ignition at speed.

Robert Leigh

[Erich]

Thank you all for the responses. This helps to explain on the other site that retarding the spark does not diminish it enough that it isn't desirable to start with the spark retarded.

[Bob Culver]

For motor bikes the range between kick start and full advance may be 40 crank degrees, but the necessary spark range for a non racer Seven is only about 15 crank degree, so unlikely to stray much out of the optimum range

Not really relevant or analogous but the curious may be interested that the advance range of the LT magneto on a Model T extends beyond one pole, so there is a narrow advance band between poles where the car will not run on the flywheel magneto. The driver learns to avoid this.