If it's not drawing fuel you either have a blockage at the carburetor (fuel nipple or spraybar) or you have an air leak somewhere in the system. If it will draw fuel when you block the muffler but not when you block the carb, you can eliminate the blockage possibility assuming the engine will run at all once it does start. I'd replace carb and needle valve o rings, then if that didn't work replace the fuel supply line inside and out. You can test the stopper in your tank easily enough by just holding your fuel line close and blowing as hard as you can into the pressure line. If the tank can hold the pressure you put on it that way, your stopper is good although you could still have a bad line inside your tank.