bob_nj,
You're right. The low end on YS91AC/FZ is an air bleed. To adjust the YS91, first adjust the high end like any 4-stroke engine, then bring it to a 2,000 rpm idle and see what it does. If it goes to idle, then speeds up and possibly dies, it is lean, turn the low end in 1/2 turn. If it goes to idle, slows down further and dies, it is rich, turn the adjuststment out 1/4 turn. When you've got idle somewhat set, got from WOT to idle, let it idle for 20 seconds, then roll on the throttle (not bang it on, 3 sec. from idle to WOT) and see what it does. If it backfires, throws the prop, and/or dies, it is lean, in 1/8 turn. If it hesitates, gurgles and takes a while to come up to rpm, it is rich, out 1/8 turn.
DKjens