The ignition fires when the voltage produced by the pick-up (as modified by a divider network) reaches the threshold voltage of the thyristor. It should always fire at the same voltage, so the timing will be determined by when the pick-up produces that voltage. The voltage from the pick-up increases with rpm, so the trigger voltage is reached earlier and so the timing advanced. The pick-up NLM supply has the magnets in the coils, rather than in the rotor and uses a carefully shaped steel rotor to develop a quite different trigger waveform which gives a more progressive advance curve.Harry wrote:I think EVGuru is getting his electrics mixed up. Advance is controlled by the resistors in the transducers, the pick up simply generates a small voltage to trigger the thyristor in the pick up.
There some good information, including in depth technical articles in German, on the Dutch Morini site;
http://www.motomoriniclub.nl/tech.html
The 300 ohm coil figure is rather misleading. It simply would not be possible to wind enough turns to get to 300 ohms with the wire size used originally on some coils, there isn't enough room and 220 ohms would be more like it.
What I suspect happened is this; The early coils used thinner wire and were approx. 300 ohm, the figure reported in the manual, but the insulation broke down and the coils failed prematurely, so later coils were wound with thicker more robust wire but less turns and lower resistance. In theory the ignition would be less reliable because there was less 'spare' voltage to allow for weakening of the rotor magnets etc. but in reality having a more robust coil is better. Modern magnet wire has much better insulation and the thinner wire can be used with no problems.
The resistance is of no real importance, only the voltage produced!
Where the resistance reading is useful, is if you know what it was when the coil was wound, you can see if it has dropped over time indicating that the insulation is breaking down.
They may have gone back to thinner wire at a later date. I've not examined enough stator coils to be sure. If in doubt I rewind them using the smaller wire and just filling the bobbin rather than counting turns.
You can use NLM's new ignition box with your existing pickup. It's really just two of the original circuits built into one box and using external coils. I know, I supplied Alex with one of my versions to experiment with. They have a rig they can test ignition components on. Alex tested three or four red pick-ups for me when I was up there last. If you know anyone who can solder, you can make your own as shown on the Dutch site. BTH magnetos can supply some neat little coils the same as NLM use.