Well, we don't know that the spring was missing, or if the op lost it while disassembling it. Also, if it was missing before the op took it apart, why is it missing? Did it run okay, then someone played with the pump and then it started having this problem? Seems like if the missing spring was the problem, the problem would have started when the spring disappeared. Maybe the spring is a problem, plus whatever the original issue is?
I would make sure the pump is right, check the gas cap vent, the fuel filter, and the entire fuel line, into the tank, for a restriction (including bugs in the tank blocking the pick-up). Then look at the pump as being weak.
"The internal combustion orchestra; sweet music."