I have 2 ST GS-40s and the way I tune them (4-500 rpm richer at max.) cause them to exhibit a similar trait, i.e., idle creeping up about 200 rpm after cutting the engine to idle from mid to high rpm on the ground.

I set the idle to 2800 - 3000 rpm. The idle is set rather rich (3-400 rpm increase in pinch test before quitting). I've never had a dead stick and I don't want any in the future.

If I set the idle to 2500 - 2600 rpm range, the dip in rpm at a sudden transition from full throttle to idle sometimes kills the engine on the ground. I have a feeling that this dip may not happen during flights, but am not brave enough to try it.

The transition is very smooth although slightly retarded than a leaner setting. I'm using 5% Powermaster and 10% Omega, both show this tendency. I used to use 15% Omega and I didn't notice this but I didn't pay as much attention, either.

I'd like to set the idle lower, but i'm afraid of dead sticks. The current setting is OK for a windy day (most of the time) but makes the landing more difficult on a calm day.

Leaner idle settings seem to make this tendency worse. Do you have any suggestions?