the fuel inlet line is at the bottom rear of the tank, filling through the muffler pressure fitting will force the fuel into the feed line as you have found out. a three line setup will let the tank vent through the muffler tap line while you fill the tank. I use a three line setup when I have a coweled engine. on an open front type airplane it isn’t necessary just fill through the carburetor line with the muffler line disconnected.