though i'm NOT an aero-engine mechanic, i'd suspect a worn camshaft.
surely a worn crank would give a noticeable mains grumble/oil-seal leakage or a pronounced big-end knock.
It is, of course possible that a big-end has "nipped" and bent the rod....that would give you vibration, but you'd expect the shells to knock out in short order.
In the absence of any report of unusual noise, starting difficulty or a severely over-advanced mag (that would "fight" the upcoming piston, would also cause vibration!) which would give a dramatic rev-drop when switched ON.
I go for poor breathing at high revs....cam lobes worn= inadequate valve opening = poor gas-flow = low power output....
Now tell me i'm totally misguided....and why!
