Chris:

If you’re getting smooth idle at 1600 rpm and sometimes 1400-1200 – I would suggest setting your idle stop to >/= 1500 rpm and always using fine pitch when on the ground – ideally, you want a combination of engine rpm and prop pitch such that the airplane won’t roll on pavement when idling without you holding the brakes. If you can consistently run up the engine and reduce it to idle without it quitting on the ground, it won’t quit when you pull it to idle in the pattern.  I had a similar troublesome idle with my first engine (’88 S4 rotors) especially when hot but it never quit while airborne.  After switching to the ’89 S5 rotors I thought I’d detect slightly more power (9.7:1 (S5) Vs 9.4:1 (S4)) but the only noticeable outcome was a smoother idle – I guess it was breathing better at low rpms with the increased compression.   YMMV … 

Jeff

EC2, RD1C, 13B-NA