Ryobi weed whackers run upside down anyway.

If you have two fuel lines from your carb, the one that is the return line has to be plugged if you do not have a fuel bulb of check valve on it.

If you just ran a straight line back to the tank and it was submerged the engine will run Ok but if the return line came out of the fuel the engine will shot off because the carb will suck air through the return line