Can you post the numbers so a member can help you, we can not see the numbers on the horn.
List-9029 and under that it says 1980
All I am finding is this could be a Marine 715 cfm, Numbers .... 9029-- 715-- (F)75,®84 8.5 4150 Marine,
From the looks of it, it's not a stock Holley. I would call the phone number on the builder tag and ask them. Who knows what it is now.
And your bog could be related to pump shot, vac secondary spring, or even vac leak. But that's fodder for another thread, lol.
There's my .02. Hope it helps. Eric L