(12-08-2017, 04:47 PM)Erich Wrote: 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

Quote:

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.

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(12-08-2017, 04:47 PM)Erich Wrote: 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