Stumbles and coughs on takeoff. What's different between static bench test and accelerating then pull up? Is the tank clunk stuck in the front of the tank? When shaking the plane around before starting do you hear the clunk sliding along the tank bottom? If memory serves me MinnFlyer recommends adding a piece of metal fuel tubing between the "cork" and the clunk to ensure that the soft in-tank fuel line doesn't allow the clunk to fall to the front during a down line.