First off the sole purpose of the fuel pump is to pump fuel to the carb, not to keep the needle in the seat. The float's sole purpose is to keep the needle in the seat, when the float bowl is full. But I digress. That is really not important.
It sounds like you could be having a problem as stated above. However my guess is that you have some build up in the carb. The carb has several jets that the fuel has to pass through. Some are for idle and some are for WOT. If there was some trash in the float bowl and you sucked it up into the idle jet it would not run at idle. If it was in the main jet it would not run at WOT. When you opened your vent to your tank did it make a sucking sound, or did it blow? If the tank was in the sun and expanded and was forcing the fuel into the carb, it might have run at idle, but not enough pressure to run at WOT. Having said that if it made a sucking sound it could be that the fuel pump was able to pump enough fuel into the carb at idle, but not at WOT. Now that the pressure is gone, your having trouble. Next I need to ask, is your filter before the fuel pump or after. If it is before, then I would say your probably ok. However if it is after, I would suggest looking at the fuel pump. I just did one that was full of trash.
See here.
So It could be a carb problem or it could be a fuel pump problem, but either way I would bet that one or both are dirty. Good luck and ask more questions