a good idle is a final indication that the carb and intake system is in good shape.. everything's gotta be perfect to have a steady, smooth low rpm idle..
a dirty air filter, a misadjusted float level, a fuel leak anywhere in the carb, any clogged or restricted internal carb passages, any vacuum leak, loose nuts and bolts, a old dried out gasket in the intake system, improper choke/throttle cable adjustment, etc, etc can prevent a good idle..
A carb has an independent idle system.. this includes some tiny passages as well as an small idle jet.. idle is also dependent on proper float level.... If the float or float needle valve has problems and float level is not steady and correct, a good idle may be impossible.
And the orifice through an idle jet is miniscule.. without finding it and poking a very thin wire through it and then using compressed air to blow everything out it may not be clean and open..
Spray some Starter Fluid around intake manifold gaskets, listening for a change in RPM to detect air/vacuum leaks.. repair or replace gaskets and use a coat of Permatex 2 (non hardening gasket sealant) to make them air tight.. check bolts all over and around the engine for proper torque ..
Idle adjustment is the very last thing that gets done after a thorough tuneup, including checking ignition timing and everything else.... and do it only on the hot engine or it won't idle when hot.