The fresh fuel coming in cools the fuel . But if you have hot bubbles coming in the float will not shut the incoming fuel off. With out a return the fuel is dead ended. The return prevents the push of the hot fuel. Believe me. This only occurs after driving it and then letting it sit for 15 minutes. If you shut it off and immediately restart it it will not flood over. But let it sit and try and start it. That is when it will flood.