What do you think happened?

Finally got some time to warm it up and check the timing and vacuum. It's all good, sitting at 20 degrees @ 800 rpm pulling 15 inches nice and steady. No vacuum leaks, the engine is happy with that timing, no pinging under load and idles well.

I'm pointing to the carb. It's a hand me down from my cousin. Holley 750 DP. Gotta be at least 30 years old. I've put in a rebuild kit, but I'm not sure it's really up to snuff. Weird things like the front passenger side idle screw has to be in all the way or it just stumbles at idle. I'll go on the hunt for a known good used or a new carb this winter. Suggestions?