Dellorto main jet size 208 suitable for the original carburettor of Aprilia RS50, RS4, RX, SX, Derbi GPR, Senda R, SM and Gilera RCR, SMT with D50B0 engine. If performance restrictors are removed, the fuel-air mixture must be set up correctly. It`s a fairly basic procedure to swap out the factory jet for this Dellorto item. Depending on the type of restrictor or performance upgrade, the main jet will need to be changed.