I have had this dual carb intake for years, Just pulled it out and did some research and found out that Jim Feuling designed this intake in the early 1990s, From what I understand this intake is more rare that the 4 valve head intake that was made with two ports for each head, this one is set up for standard heads with a carb on each side of the bike. One carb runs normal and the other carb is progressive, It opens at mid to full throttle.