Ernie, that is the Saito backplate pump to pressurize the tank a little, it comes with the large carb, the needle valve regulates the tank pressure by restricting the crankcase vent flow, the excess just bleeds off in the normal way. The thing is like a PolyChoke on a shotgun, evertime you shoot at something and miss you become convinced that you don't have it set right. The large carb works very well with the Cline or IronBay, I have not preferrence of one over thae other except in the case of my Saito .30s, the Cline will allow them to idle much slower than the IronBay. Any engine larger than the .30 and they work exactly the same.