Originally Posted by FullOppositeRudder
I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one.
The vibration occurs over a wide rpm range, but only above a certain figure. The onset figure was increased by making the engine run smoother. Most likely this is propeller vibration. The normal prop vibration band for this aircraft/engine/propeller combination is 2250rpm to 2550rpm. With the engine running rough the onset would be at a lower rpm.