That plastic deal is an insulator to help keep the carb from getting hot and causing vapor lock. So yes, it is useful. I've seen two types, one with just plain holes in it with no restriction, and one with a sort of drop in cylinder of sheetmetal that makes the air go deeper into the intake before turning toward the cylinders. You could take that metal part out, but I figure it probably does something to modify airflow across the secondary barrel of the carb. I imagine they found some of the air from the primary backing up the secondary a bit and tried to get the primary airstream deeper in the intake or something like that. I've never noticed a difference running with or without it though, one way or another.
Yours sounds like it might be a little different. Hard to say just why they would block airflow, so it probably had some theoretical purpose. If it is as bad as it sounds, I would probably replace the plastic part with one of the older plain hole style. The plastic insulator is helpful in keeping the carb from boiling off fuel, but restrictions to air flow are seldom helpful.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
