I looked at a couple of your earlier threads, and in particular after you switched to the '68 dizzy you mention that you still had a 'slight miss'. Is that still the case?
I also looked at your thread about purchasing the car and you mentioned it had been sitting for a long time. A common problem that occurs after a long period of sitting is valve(s) sticking. It happened to me. I discovered mine when I was trying to track down a miss on my current engine after a long sit. I pulled the valve cover and mine was easily detected because one pushrod had bent sufficiently it actually worked it's way out from under the rocker arm (kinda jammed in there pinned against the rocker).
The valve was stuck partially open and there was a miss throughout the range, but no perceivable valve train noise accompanied it. The miss was more noticeable at low idles than at higher idles where the inherent balance of an inline makes something like that less perceptible to me. I pulled the rocker arm assembly and gently worked the valve loose with some very light taps from a small hammer. I put a dab of lubriplate on the stem where it meets the guide, manually worked the stuck valve open several times, and put in a new pushrod...that took care of it, with no recurring issues.
EDIT: Still runs and drives great today...touch wood
A compression test wouldn't hurt and can also help indicate a stuck valve (low compression on that cylinder), but visual inpsection is quick & easy.
{EDIT 2] A rythmic/consistent miss throughout the range is generally indicative of an ignition [or valvetrain] issue. I'd pop the valve cover and check the valves for full operation. A miss could make it difficult to get smooth idle. Never hurst to double or triple check the firing order...BTDT one too
Otherwise, as far as the idle circuit itself goes, if the valves all check out I'd wonder about the condition of the carb, has it been rebuilt recently? Yeah, like lots of folks I've gotten by timing by ear, but it's always best to check it with a light (and a vacuum gauge if possible...lights can fail you on these old engines if the balance has slipped...moving target). Good luck!
