A monofuel supply system (4) for an endothermic motor (32) powered with alternative fuel such as LPG, methane, hydrogen and the like, comprising a flow adjustment device (12) having a device body (16) provided with a main intake pipe (20), at least one shutter (24) intercepting and occluding at least partially the main intake pipe (20), and a delivery pipe (28) located downstream of the main intake pipe (20), a tank (36) of alternative fuel, wherein the main intake pipe (20) receives a main flow comprising a supporter of combustion and an alternative fuel, coming from said tank (36), mixed according to a first titre (al). Advantageously, the system (4) comprises a secondary intake pipe (44), for inputting a secondary flow, different from the main flow, directly into the delivery pipe (28) bypassing the shutter (24), and is provided with an enrichment diffuser (48) of alternative fuel, in fluid connection with said tank (36), so that it enriches with fuel the secondary flow and sends it to the delivery pipe (28) with a second titre (a2) which is richer in fuel than the first titre (al) of the main flow.