I too have a IO-375 with dual Pmags and the FM-150. Idle was unstable and hard to adjust. I found three issues and things are much better now - still not perfect however:
- the diaphragm in the flow divider had a leak from the very beginning
- the spring in the flow divider was installed incorrectly so that I always had
max flow through it.
- I installed smaller injector nozzles, went from .028 to .022.
Might be worth to have a look into this.
RV-7 flying, IO-375 w. P-Mags, 200RV Prop
RV-8 flying, IO-375, P-Mags, 74RV Prop