gas leak

I got gas leak from filter box . did befor so i have a hose on the nipple so wont get on the engine. thought was something from carb got carb cleaned still leak so found out it was o ring in reserve switch think its called pilot . worked fine for good wile now gas leak agen and i see black stuff drip from bottom of muffler where connected befor it go's to the 4 pipes i thank its oil. what could be the prob.............