I have a couple of questions, is this on gas or alcohol? Does it have it extensions on the back? If it is on gas it could be sloshing fuel out of the vent tubes and casing a really rich condition, needs tubing on the vent tubes. Maybe it is the fuel getting away from the back jets. If it is alcohol does it have .150 needle and seats? Just some things to think about instead of the pump. We always run our return style regulators after the carb so you don't lose flow.
Curtis