Sounds like the tank is mounted too high above the carb.  The only way I know to deal with this is to use a hemostat to pince the carb fuel line while you fill the tank.  Don't release the hemostat until you're ready to start the engine.  Don't open the carb past idle until the engine is running.  Only other option is to fuel the plane inverted and turn the plane over when you're ready to start.  Both options are a pain, but about all you can do without lowering the tank.  A two line system will not solve the problem.

pmw