I agree. Long duration and low vacuum isn't ideal conditions for eddy carbs because vacuum dominates the metering rods. You may get it closer by playing with the springs, but still. I would move to a Holley style carb...
I put the lowest springs which stayed closed at idle and eventually removed them the springs for troubleshooting purposes. If i create a vacuum leak at the base of the carb it leans out the air fuel mixture and idles great.