Abstract: A transmission unit for self-propelled vehicles, comprising two pulleys, i.e., a driving pulley and a driven pulley, with variable pitch, connected to each other by a transmission belt, is provided with an engine start-up device with an actuation group acting downstream the driven pulley; such an actuation group causes, at start-up time, the jockey element to act on the transmission belt, for the purpose of recovering possible slack of the belt; furthermore, lock elements, disengageable by centrifugal force, are provided, which mutually constrain, at start-up time, the two half-pulleys which compose the driven pulley, for the purpose of preventing the belts from wedging themselves between the same half-pulley.