Idle speed is controlled by spark timing and throttle opening. A vacuum leak is an uncontrolled source of air. The fuel mixture has to be rich enough that a vacuum leak only results in an increase in idle speed. If the leak is minor enough to raise the idle speed without affecting performance in other RPM's, it is easily found using soapy water to spray around the carb holder where the carb pushes into it and where it seats to the cyl head.
Other causes of uncontrolled idle; most common - not enough slack in or sticky throttle cable, not enough slack in or sticking enrichener cable, dirt, corrosion or debris preventing the throttle butterfly from closing against the throttle stop screw (idle speed screw) - less common - a perforated air cut valve diaphragm, high float level, throttle butterfly that has loosened and not closing completely due to binding in the venturi or damaged butterfly shaft, missing or incorrect size air jet in the bell of the carb - more obscure - a bent slide needle or damaged pilot screw orifice.
Good luck - it takes a good eye to see some of the less common to obscure causes - vacuum leaks are easy
Shop Owner and Mechanic with over 50 years experience