I did the same thing on my '03 EC250, and your right it advances quick and stays there until the revs drop. I was trying to see if I could tell a difference between the OEM CDI and one from an '00 XC250 that gave better low and mid power. I could not, using this crude visual method, but there was definately a difference in performance. I did not have a tach, and I did not rev the bike full out. I think what appears to be an instant advance is actually a curve but one with limited resolution over a narrow RPM bandwidth. It would depend on the processing power of whats in the CDI box. Also, with no load (I'm assuming you just reved the bike in neutral like I did) its easy to blow through this RPM range. With the bike on a dyno with a tach you could probably learn a bit more. The trigger pulse is actually received by the CDI at or greater than max advance (deg BTDC), and the box acts as a variable delay device.
If I could power the CDI from an external source, and had the time to do so at work, it would be fairly easy to set up a test rig with a PC and some I/O hardware I have to trigger the input and wait for the output, at the full range of simulated RPM, and plot the time delay (advance curve).
Theoreticly the curve should advance up until the peak power and then retard in the "overrev" range. If your interested in this Google it there is a lot of info especially related to tuning 2-stroke road race bikes. Its nothing like a 4-stroke. Interesting stuff.
BTW, the CDI with the multi-pin connector is the newer dual map unit, used on some '06s and up.
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