Chances are you knocked something loose when changing the filter or your Q-jet and lodged it in the needle and seat. Q-jets like very clean fuel.
As far as fuel filters before or after the pump, after is always better, but if you install a bigger filter before the pump and keep it clean, it won't effect them at all. Another thing you have to remember is that from tank to pump is a fairly short distance on a truck, so the pump isn't really working that hard pulling fuel. Now if the tank is way out back, or you have a thirsty big block drinking loads of fuel, then yes, you need to move the filter.
If its already in place and working, replace it often or till it clears up, install a filter before the carb inlet and enjoy the drive.
Joe
