You can't blow in the supply, the inlet needle takes more pressure than you can proved to move it off its seat....
Doesn't need a return line to the tank..Some chainsaws and garden equipment use the return...If there's a small tube in the side of the carb you can blow in, plug it. It was connected to a one way valve on the equipment the carb was made for..The carb won't work right with an air leak in the side..