carb idle

I think TB nailed it.
The clue is that the idle hesitates high before falling. I'm guessing the throttles are too far up the transfers, cuz you have screwed in the curb-idle screw in compensation for a failed low-speed circuit. Either the emulsion tubes are plugged, the pick-up orifice is, or most likely, the air-bleeds are plugged.
The hanging idle could also indicate sticking flyweights, but follow TB's advice first, it is the more likely cause.