If everything else is okay the fuel should drive out the air or vapors unless the fuel lines are rediculously hot.

A few things, some obvious some not. Leaks either in the line itself or on, or in the fuel tank could cause that. Usually you will see excess fuel but a minor line leak may not show fuel. If the cap of the tank is not sealing right that could cause the problem.
You didn't say if it happens when the tank is full or part empty. Usually a cap not sealing will show up when the tank is not full. Change the fuel lines, and route the lines as far from the exhaust pipe as follows.
Is the engine running hot? If it is and the jets are set rich, chances are their is a leak. Change fuel lines first, cheap and quick, change gasket on tank.
If not that, then check for leaks on the engine. You can block the exhaust port on the engine, pipe off and block the intake with soapy water all around carb, back plate etc. Blow into the new clean fuel line and see if you get bubbles. If you do, seal the engine with nitro proof gasket material, check the threads.