pull the motor
put the piston a bdc. open carb needles, put a piece of fuel line on the carb fuel inlet. cover the carb intake and exaust port with your fingers and blow into the fuel line, have someone with a small modelers paint brush ,brush soapy water on your motor and u will see any leaks. make sure to check the block also. i have seen one with a cracked block do the same thing.
427 is a very good motor and easy to tune.