I have been pulling my hair out trying to figure out the idle issue on this 68 I'm working on. I finally swapped the one from my 60 to the 58...voila! It runs perfectly with the known good carb. I pulled the top off the Carter and it looks fine...I even double/triple checked the gaskets under the carb itself. Where else in the carb could I have a vacuum leak?? It is bad enough that the car will barely idle when warm, and it will not idle when cold without major choke. I am not a Carter guy, so my knowledge is very limited. It is a fresh rebuild, too.