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Sean;

It does sound like a possible intake leak, but normally those don't settle back down...

Is this a recent and new symptom, has it always done this in this configuration?

I'd look at throttle linkages (especially return springs) to assure linkages are well lubed and throttles are returning to idle position. Is fast idle somehow interfering?

If later carbs with (shaft mounted throttle springs and throttle overrun valves) I'd check to assure neither was hanging up...and while were at it, if dual thottle (late) manifold, check secondary throttle shafts work as expected, then recycle that POS manifold (at least the intake part!), and replace it with an alu manifold.

Good Hunting!