Holley idling issues

Make sure your idle speed setting is a good ways AWAY from anything happening advance-wise in the distributor.

A bad example: Your idle gets you 6" of vacuum and your vacuum canister kicks in at 5".

Bad example #2: Your idle is set to 800rpm and your distributor mechanical starts at 900rpm.

Centrifugal start should be a good 3-400rpm away from idle...so I've been told by people that should know, anyway.

Also, as mentioned, check for slack in your throttle cable. I chased a hanging idle for a LONG time before I realized my cable was a tad snug. I had not changed anything that I remembered. Maybe I had reduced my idle on the carb and NOT checked the cable to make sure I didn't catch up with a bind? SOMEthing happened, and a too-tight throttle cable was the result.

That it 'used to be' good doesn't mean something simple is NOT the problem, some little change made maybe.

Good luck.