A good selection of accelerator pump discharge nozzle sizes is indispensable to the proper tuning of the accelerator pump system. The nozzle hole size determines the rate of fuel discharge. a larger hole will "shoot" the fuel at a faster rate, and with less pressure, than a smaller hole. Sizes are stamped on each nozzle. For example, the number "31" indicates that the nozzle hole size is . 031". Various styles of nozzles are offered by Holley, including the following: Used on Models 4150, 4160 and 4500Pit Stop USA offers Holley Tube Type Accelerator Pump Discharge Nozzles - .040" 121-40 at low everyday prices. Be sure to check out all of our products from Holley Performance Products including Holley Performance Products Pump Discharge Nozzles.