If the Walbro has a prime pump, it is normally (not on all models, see below) used to prime the carb with fuel only, so it is ready to rumble.
To get this done, the carb pump needs a fuel return line to the tank, which should be submerged at all times.
On some carb models the prime bulb not only serves the purpose of filling the carb with fuel, but also to prime the engine as well. I have read this in their patents, but do not know which carb model incorporates this idea. It would replace the choke valve or "wet thumb system".
Maybe RCign knows?