 The port valve is therefore effectively timed to remain closed whenever the secondary carburetor throat is closed, isolating the induction tract from exhaust gasses.Carburetor tuning involves adjusting cables controlling the primary butterfly and the port valve, among other things, illustrated by Suzuki Service Bulletin Nine.The carburetor also incorporated a fuel pump which was mechanically actuated at 35 degrees of primary butterfly movemement (later changed to 28 degrees) to enrich the fuel mixture during acceleration.