Can someone tell me where did this Holley 1920 come from?

cudamark said

I'm trying to figure out how the bowl vent is supposed to work on that carb....

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Just like a percussionist's high-hat cymbal. There's a link rod that runs vertically up thru the carb body (not thru the float bowl) and lifts the "hat" valve off the bowl vent when the throttle is at idle position.