Those who have done the test, please chime in.
Richard L.
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A magneto needs sufficient magnetism to work throughout the rev range. The benefit of very good magnetism is felt at very low engine speeds, particularly kickstart speed. The function of the magnet is to energize the low tension circuit which, in turn, energizes the high tension circuit through the usual principles of an induction coil. A good magnet will saturate the soft iron laminates of the ignition coil core so that, as the coil turns through 180 degrees and its polarity changes, the primary windings are fully energised. If the magnet is weak it will only generate a low voltage in the primary circuit and, consequently a low voltage in the HT circuit. A magneto tends to work better at higher speeds (within reason!) in any case so, unless something is significantly wrong elsewhere in the magneto, the issue of magnetism is mainly a low revs issue. I read somewhere that you need 18000 volts to jump a 6mm gap in normal atmosphere (I may not have recalled this correctly and I'll try and find the article again). All K2F's, in decent condition, will throw a spark 6mm when spun at hand speed. Most of us run on 20 thou spark plug gaps and a compression ration of about 8:1 so the magneto is more than capable of doing the business. A couple of years ago I built a magneto, out of parts, for my own bike (K2F, automatic advance and retard for the A10) and did not remagnetize the magneto. At a petrol station in Leicester, returning from the Ace Café, I was turning the engine over on the kickstart to find compression and the bike started! This is anecdotal and proves nothing (except that maybe the ignition is a bit retarded!). We know from experience that if you don't kickstart a bike hard enough it will kick you back; this proves two things 1) The magneto is working! 2) insufficient momentum has been created in the flywheels to overcome compression (and maybe the ignition is overadvanced). If we're not careful, when discussing magnets in the context of magnetos, we can get into the realms of 'overegging the pudding'. There are two principles to observe, 1) If there is enough magnetism to start the bike at reasonable kickstart speed the magnet is fine. 2) If the magnet is already fully saturated no amount of remagnetizing is going to make it any stronger. But, of course, a fully saturated magnet is going to make the bike easier to start, or facilitate starting with a bigger plug gap, than a weak magnet.
With reference to previous comments about magnet strength I have been testing the magnetism of a pile of magnetos from the workshop shelf. I have a gauss meter which works on the same principle as the compass needle test previously described. What I have found is that the magnets on K2F's all deliver a strength of magnetism within quite a narrow range, let's say 8-11 cm to achieve the same needle deflection. BTH magnetos seem to be a bit weaker and fall in the range 6-10 cm. After remagnetizing, all the magnetos achieve maximum strength, or marginally more, which suggests the magnets are saturated. I am prepared to bet that, although the measurements suggest some magnetos have lost over one-third of their magnetism, they are still capable of starting an A10 quite easily.As an experiment I intend to build a refurbished magneto from the weakest magneto (magnet) I can find just to see how it performs on my A10..... Any thoughts?!
One further point is to remember that magnets in K2F's are made out of a completely different material to the old soft-iron magnets you see on veteran bikes which are renowned for losing their magnetism and need to be recharged every few years. The magnets in K2F's are described as 'permanent' magnets for a reason.
