Any idea what causes this?

put a rag on top of the carb slowly .does it pick up speed or lower speed /if it goes up its running lean (needs more fuel the idle circuit is restricted or there is a vacuum leak somewhere ) and most of the holleys rebuilds are junk they dont take the metering plate apart to clean it they just soak/blast them .the tiny fuel passages are never open up.I have been dealing with one like yours for 3 months. also there is 6 different accelerator pumps for these models they are very close but you may need to adjust the accelerator pump linkage . the one i bought was just bead blasted and new parts/wrong parts/wrong adjustments and sold . good luck