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Presuming everything else is set right (spark and cam timing, valve clearances, etc), sounds like it’s a chronic lean condition at idle. Most likely cause is a vacuum leak - ie. intake manifold to head, carb to manifold, worn carb throttle shaft bushings, etc. To find the leak, with the engine idling and the choke full open spray WD-40 or play gas from an unlit propane torch on the suspect areas. The idle smooth out and speed will increase when the WD-40 or propane gets sucked into the leak. If the above doesn’t do it, the carb idle mixture is chronically lean. If you slowly pull the choke with the engine warm and idling and find a choke setting where the idle smooths out, that’s the problem. You’ll then need to go back inside the carb and look for a partial blockage in the idle circuit, a wrong (too small) orifice someplace, a crack in a casting (unlikely), etc. Good luck! Cause for a problem like this can sometimes be tough to locate…
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