Why is my car idling too fast?

Either the idle screw on the carburettor is improperly adjusted, or you have a vacuum leak.

If you adjust the idle screw as far as possible (usually, a quarter turn or so is plenty) and the idle is still fast, it's probably a vacuum problem.

Check all the vacuum hoses and connections, and around the gaskets for the carb body connection to the engine or the intake manifold. If you can't identify an obvious leak (sometimes you can hear the whistle from a hose with a split), get a carb cleaner aerosol and spray it around suspect areas. If the idle suddenly runs more smoothly (due the fact the aerosol has stopped the oxygen getting into the wrong place), you've found the location of the vacuum leak.