Have you ever removed the float bowls?
I had a holly 750 that I bought used and had the same problem. It turned out the plating on the inside of the carb was peeling off the inside of the carb, I think the term holly used was diechrome plating?
Anyways I contacted holly and even though the carb was 20yrs old they sent me a Brand new carb, no charge.
It would pay to pull the bowls if you haven't and check it out.