A mixture preparation device for double-feed engines having a body (11) in which two ducts are provided, namely a first duct (12) for air feed to the crankcase and a second duct (13) connected to an injection device. Throttle elements (10, 35) are provided for the two ducts (12, 13). In the second duct (13) connected to the injection device there is provided an air-fuel mixture emission unit (29-34) adjustable in its degree of opening, the opening being controlled by transmission elements (49, 50) operationally connected to a shaft (36, 43) carrying one of the throttle elements (35) positioned within the first duct (12) for air feed.