Another remote possibility that you have probably already checked is a pinched silicon line, which might cause a loss of pressure to the tank, but you would think this would show up as more than just a surge at full throttle. Just a thought. If the exaust is the problem, like Phoenixangle said, you could take an ink pen or small marker top and shove up into the tube and ziptie it in place, then drill a hole in the end of the pen top to adjust the diameter of the opening till you get what you want.