Idles too fast

The throttle on your engine has a spring that would hold the throttle plate fully open if the cable was to break.  This is opposite from an automotive engine that has the throttle sprung closed.  You can understand why this is, if your cable broke during flight you would want the engine to continue to produce power until you reached an airport, where you would do just as you described, turn off the engine once the runway is made. 

Because the throttle plate is sprung open, if the cable slipped a bit at the clamp on the throttle body, then it would not be able to pull back the plate to the full idle position.  Also, typically there are idle stops at both the throttle body and the throttle lever itself. The idle stop screw on the 912iS throttle body is factory set and does not typically need to be adjusted.  It is installed with Loctite and typically has torque seal on it so you can see that it has or has not moved. It’s important that the stop screw on the throttle control lever needs to be adjusted so it touches just slightly before the idle stop screw on the throttle body.  If this is not done, and the stop screw of the throttle body hits first, then you will stress the cable and the throttle body control lever every time you pull the throttle back to full idle.

So either the stop screw on the throttle lever has come loose and moved forward, or a cable clamp has come loose at one of the two ends, or the cable is failing (which would be rare). Don’t just adjust something.  Inspect everything carefully, until you figure out what happened. Remember that any adjustment you make needs to leave the idle stop screw at the throttle control lever touching first.  

You’re exactly right and that was the problem.  Fixed.  The cable was slipping at the end.  Does several attempts to get right.  Now idles at about 1700.  Power all the way back on descent still shows about 2100 rpm, falling to 1800 or so on rollout.  Best it’s been in the year that I’ve owned it.  Thanks.