it sounds like you have the gas line hooked to the carb. if that is the case, it sounds like your in-line shut off valve is either not closed all the way or is leaking internally. (you do have a shut off valve on the fuel line right?)
it could also be that your float in the carb is stuck down. remember, these are gravity feed, and if the float is stuck down, the needle & seat in the carb will always be open causing gravity to constantly drip gas into the carb. eventually, gas will come out of the carbs. overflow tube, or the back of the carb...whichever path it can find to over flow. if your float level is set right, and you leave the shut off valve open, gas can still overflow slightly as the air temp. changes, gasoline expands and contracts with the temp.
when the float bowl gets full and the needle & seat doesn't close to shut off the fuel supply it will just keep dripping until your tank is empty. you should install an in-line shut off valve in the fuel line if you do not have one.
i suspect that this is the problem.