Sounds like possibly an intake air leak, could be intake gasket or dry rotted transition piece between intake manifold and carb. If the leak is small enough or in the right spot, after the engine gets warm, things start to expand (metal expansion can seal the intake gasket leak if its not too bad). Try starting it with a "shot" and before it warms up too much, spray WD40 thru the little red straw in the area of the intake manifold gasket and other possible leak points to see if you get a noticeable change in rpm. Changes in rpm indicate a leak in the area you are spraying. Vacuum lines tend to harden up with age (due to heat from engine) and lose some of their sealing ability. Replace them if they are not soft and flexible. Check or retighten all bolts on the intake manifold and carb.
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