Update - I've finally been able to run the engine again and try both tachs.
The used one I've bought works fine at all rpm in the plane, so I do have a solution :-) and it says there's nothing wrong with the Rotax signal.
The original one is accurate up to 2000rpm. As you pass 2000, with barely any change in engine speed, it jumps instantaneously to indicate 3600rpm. As you increase engine speed further, the error increases so at 3500rpm the tach is going off the scale at 7000. As you slow, you get the reverse drop from showing 3600 to 2000 as you get to 2000.
I tried several times up and down and the behaviour is consistent every time. The needle move is rapid and one jump. Other than this it moves smoothly and steadily and doesn't vary at constant rpm.
Remember this tach fed with 50 Hz from a mains transformer indicates a steady 3000rpm. Fed with the Rotax output at a genuine 3000rpm, it steadily indicates just under 6000!
For curiosity I'll try it with the loading resistor but I think the tach must have a weird fault that the guy I bought the plane from "forgot to mention."
