I must be missing something here. How does a plugged return fill your engine?
You have fuel line gunk in be carb making the n/s not close or a bad tank vent.
Because your engine doesn't use all of the fuel that's supplied by the fuel pump.. Pull the throttle, engine takes in air due to increased vacuum, vacuum draws fuel into the carb, jets regulate the amount of fuel into the engine, and the excess goes back through the return line. If it's clogged, the excess fuel has nowhere to go but down the intake tract until the motor fuel-locks
At least that's what I think. Someone correct me if I'm wrong please!!! Google or the x couldn't answer the question so I just thought of that off the top of my head..