Hi my friend
I'm not an expert, but let me share first some thought about the timing thing.
A variable reluctance (VR) sensor is very simply a small magnet with a coil of wire wrapped around it. The magnet creates a field, and any changes to this field are sensed by the coil of wire. In the figure above imagine the magnet inside the coil. As the magnet is stationary, what will you see on the ammeter? The answer is zero. No change of magnetic flux when the magnet is stationary = no induced current. However, what will happen if you move a ferromagnetic material (iron, steel) close to the magnet? The steel part is attracted to the magnet, which in physical terms means the magnetic flux is distorted towards the steel. As you move the steel part close to the magnet you induce a current in the coil. Providing you keep moving the steel part at the same speed, the closer you approach the magnet, the higher the flux distortion. As an aside, you can feel this when you bring any magnet close to an iron part: the closer you get, the higher the force trying to pull the magnet onto the iron.
Similarly, when the steel part is stationary (either close to the magnet, or far away), there is no change of magnetic flux, therefore no induced current.
Relating this to our flywheel, you have the sensor mounted close to the flywheel, and the steel strip passing the sensor periodically.
Before the steel strip arrives at the sensor, the gap between the flywheel and the sensor is constant = no flux change = no induced current; as the steel strip approaches the sensor, the magnetic flux is distorted by the steel approaching so we begin to induce a small current; as the steel strip get closer to the sensor, the flux is distorted more, so the flux change is higher, and the induced current is higher; as the leading edge of the strip is passing the end of the sensor, the magnetic flux is disrupted to the maximum (the radial gap is closing, and the disrupting influence of the edge is passing through the strongest area of the magnet's field) so the induced current reaches a maximum level too; after the leading edge passes (but before the trailing edge approaches), the gap between the strip and the sensor is smaller than it was, but importantly it is constant, so the only influence is the retreating edge passing away from the magnetic field. As the edge retreats, the induced current rapidly falls to zero.
Now, as the steel strip is going past the sensor, the radial gap remains unchanged, and therefore before the trailing edge arrives the induced current remains at zero.
As the trailing edge comes close to the sensor, the magnetic flux that was being attracted to the steel begins to be less attracted. The flux is thus changing and therefore a current is induced once more into the coil. Note that now the flux is going from a high level to a low level, so the direction of the induced current is reversed compared to when the steel strip was approaching the sensor (when the flux was going from a low level to a higher level...).
The rest of the story is as for the leading edge, but in reverse...
So in summary you get an induced current that increases from zero to a maximum, then back to zero at the leading edge of the strip; then in the opposite sense from zero to a maximum and back to zero at the trailing edge of the strip. The geometry of the sensor and flywheel and metal strip make the waveform approximately sinusoidal in both directions, and of course the polarity depends on which way you connect the sensor. If you connect one way you'll get a positive waveform on the leading edge; reverse the connections and you'll get negative waveform on the leading edge.
When the trigger signal arrives at the CDI, the circuit integrated into the unit calculates two things:
• the engine speed ;
• the required time delay after seeing an edge of the trigger pole before making the spark.
This can be done either with analog or digital electronics, the second of which is most common these days, and certainly the easiest to understand: the measured time between two pulses, or one engine revolution, generates a value used in a 1D 'look-up' table stored in the processor memory; the value from the table is in effect the ignition delay.
These calculations are typically started when the CDI receives the leading edge trigger pulse (let us assume this is positive), and are finished well before the trailing edge pulse (the negative part of the trigger signal) arrives. So the CDI processor, having finished the calculation, sits waiting for the trailing edge pulse... when it arrives, it counts the required time delay, then releases the capacitor charge by a transistor (a switch) through the coil.
Now, moving to the specific variables:
• The length of the trigger pole (metal strip) is critical.
• The position of the trigger pole relative to TDC is critical.
• The position of the trigger sensor is also critical, but only relative to the position of the trigger pole.
Sooo ... that advance that you are measuring is at idle speed ?? Do you remember I've told you I would make the cover in a way that I could move the pickup coil ?
I don't know in this case the advance at idle, but I wouldn't mind that now, I would look to the max advance and tune it to stay below 40º advance to be safe, and then look at the idle advance. This is with a cover that I could do that, now you will have to figure out the way to do it with your programmable ignition.
About the stator let me try to figure it out after some sleep
Hope it's not a too long message
Cheers
ZAGA
