I cannot explain it,but those clear filters can be misleading sometimes. I know it seems like they should run absolutely full of fuel,but I have never seen one do it. I know it sounds impossible,but that is just my experience. Can you tell if the stumble was a lean-surge or a rich-bog? Also,ignition issues can closely mimic fuel delivery issues. It can be hard to tell the difference at times. A vacuum leak could be the culprit, as well