sounds like the transition slot is keeping it running and as you drop the idle speed down once you get close to idle speed (and less of the transition slot is below the butterfly feeding fuel) it goes lean and dies.

i would turn the idle MIX screws out to richen it up (a good initial setting here is 1.5 turns out), start it and then drop the idle speed down to where it needs to be. then slowly turn the idle mix screws back in to find your leanest idle tolerance.