Classic/Antique Car Repair: 53 Packard Patrician, vacuum leaks, manifold gaskets

Question
wont idle when warm when cold perfect,did valves and intake manifold gaskets,no vacuum leaks pressure is about 16 lbs was told it could be a valve guide expanding when hot ,was told to try leaded gas and drive it on the highway,put some seafoam in gas tank as suggested,have you ever heard of this?also tried a different carb that was perfect on a another car,give me your thoughts Pat

Answer
A valve guide expanding would cause a skip more at high speed than at idle. And of course I have my own feeling about additives. They are great for making the seller and manufacturers rich but they still have not perfected the art of putting a mechanic in a can. When the engine is cold the automatic fast idle keeps the engine idling high but when warm the idle drops back. Now I would begin with vacuum leak testing. I use spray carburetor cleaner sprayed around anyplace that a vacuum leak could occur. You may have to set the idle up to keep the engine running. Now if no vacuum leaks are found pull the carburetor off. Check down in the bores of the manifold. Some of these manifolds have steel sleeves pressed into them to separate the intake from the exhaust. I have seen these burn through and cause the problem that you describe. Keep me posted. I have more thought but check these items first.