The accelerator pump dischage valve (also called the intake and exhaust valve) bleeds off some of the accelerator pump output back into the float bowl. This controls how much fuel is actually delivered to the pump jet. Once you have selected your accelerator pump jet size, you can use this valve to fine-tune the output of the pump. If the accelerator pump jet is a little too large, one of these valves can get you to a setting "in between" jet numbers.
Discharge valve sizing, similar to Weber jet sizing, refers to the bleed hole size in .01mm increments. The 40 discharge valve bleeds fuel back into the float bowl as if it had a 0.40mm hole. The 100 size bleeds like a 1.00mm diameter hole, delivering the least fuel to the jet. (The 100 size makes a 50 size accelerator pump jet behave more like a 46 size.) The 000 size has no bleed hole, so it forces all of the accelerator pump volume to the accelerator pump jet.
The closer your accelerator pump jet is to being too large, the larger the discharge valve required. The closer the jet is to correct, the smaller the valve required.
This valve is used in the DCOE and IDF series, as well as both the 48 IDA (Dual) and the 40/46 IDA (Triple) carburetors. One required per carburetor. Sold individually.