Mercarb is a copy of the Rochester 2GC and the problems encountered are similar. Fuel dripping from the booster at idle is either a vacuum leak or plugged idle circuit. Check carb flange gasket for correct fit. Remove air mix needle and blow out that circuit. Remove the venture booster and blow out all air bleeds in the booster and the main body. If that does not correct the problem then the carb could be over filling. Check the fuel pressure (like to see 4.5 to 5.5lbs max) and float level. Sometimes a back flush before doing any of the above might help. Start the engine and bring up to temp. Rev up the RPM to 2500 or so and cover the air intake with a gloved hand or rag while leaving the at same 2500 or so position. The engine will begin to die, but just before it dies let it recover. Repeat this several times. What happens is fuel goes through air/vacuum circuits and visa versa. If the idle circuits are dirty this often flushes them out! Good Luck
Another good possibility is the main discharge check ball may not be sealing. Remove the venturi, T & spring. Using a drift punch tape lightly on the check ball to seat it.