Couple things, if the new spark plug was a different heat range, it would affect the idle characteristics. Second, the idle air screw having trash or corrosion will do this.
For those that don't understand carburetor fundamentals, the carb runs off the idle air screw from idle to about 2500 rpms. From about 2500 to 3500rpms, it's a combination of idle air and main jet fueling. After 3500 rpms it's all main jet fueling. So, hopefully that helps some folks narrow down their carburetor troubleshooting.
For example, idles fine but boggs down at higher rpms, you have main jet issues (the big one that pokes up through the middle of the carb). Bad idle but runs with open throttle, idle air issues (usually a screw or smaller needle type, may be under a thin tin cap that you drill or pry out (EPA thing), up in the front of the carb).