If the engine runs smooth and pulls good under load but breaks up under no load I'd first suspect a vacuum leak.
Check for loose carb mounting and intake manifold clamp bolts. If those are OK, run the engine at idle and play an UNLIT propane torch around the gasket gasket areas. If the engine smooths out/speeds up when playing gas near a gasket it means there's a vacuum leak there. Time to replace the gasket.
Check also for a hairline crack in the intake manifold. Not common but it does happen.
Another possible cause is a bad carb adjustment (excessive idle air flow) or a plugged idle mixture jet.
Suggest also checking the ignition timing. It should be at TDC low idle and advance to 35 - 40 deg BTDC by around 1,000 RPM.
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