Hi. I can tell you that on my old carb - 62 Rochester one barrel - the throttle shaft was worn and sloppy and the carb would suck air through there. Set the idle in the driveway, running great, stop at a light a little later and it dies. The throttle would be closing in a different place. Reset the idle, runs fine a bit, stop at a light and now it idles too high. Throttle is now closing back where it was before. Doesn’t take much variation to throw off the idle. On careful inspection, I could see what was going on. I solved this when I located a NOS carb on eBay. I think you have to watch out for the rebuilt ones as sometimes they aren’t any better. Possibly you could drill out the bores and add bushings, or make some kind of seal to reduce the air leak.
