It looks like you have some crud in the idle system on your carburetor. Nothing short of a rebuild is going to help, although there's one trick you might try first. Get the engine running at a pretty good speed, 2,000 RPM or so, and either close the choke plate completely, or slap a big piece of rag over the top. This allows the manifold vacuum to suck liquid gas through all the passages in the carb, and sometimes it will flush minor deposits out. Several of these maneuvers will clear things out at times, but don't count on it.

Send an IM to Carb King here on the site. He runs a very good carburetor rebuild shop, and he can probably rebuild the one you've got (best choice) or sell you a quality reman carb. Swapping to a newer carb or just dropping any old 4GC on it would be a terrible idea. The 409 was a pretty limited production run, and the chance of another type or part number carb being right is slim to none.
Jerry

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