This is a photo of a fouled CPS center-magnet. Note all the "hair" of metal filings on the middle magnet. This both weakens and 'spreads out or shifts" the center polar point.
This is what it should/will look like if "new" or after it's just cleaned off with a damp rag, the magnet is very weak, you don't even have to remove it to do this.
I apologize for the blurry pic, but you get the idea... it can't/won't work properly with all those metal filings stuck on the magnet. On the inner 6X side of the sensor, wind from the rotating vane causes the spark to advance (on the highway) but as this wind diminishes (lower RPM) the magnetic "hairs" (along magnetic field lines) straighten up and the filings across the top and sides rejoin with one-another, weakening the overall magnetic field. This ironically results in almost the same ignition problems as pitted points and a bad distributor cap do, now easier to remedy but much harder to get at.
