A small drain tubes is coming from the Carb bowl drain (which screw may be not be shut off completely). More likely, it comes from one of your two drain tubes coming from the Crankcase emission system connected to the airbox. If your a carb float needle is not shutting off tight, because of wear or some dirt particle, gas will overflow into the airbox and drain trough the crankcase breather separator.
Once you have cleaned your carb bowls,and replaced bowl gasket, it would be wise to change your 30 year old rubber gas line and insert a basic gas filter between the tank and the carb, to prevent further dirt, bubber or rust particle to force you to redo the same cleaning operation again and again. I had the same problem with my snow blower last fall, same cure.