A purifying system for combustive air in an internal combustion engine is provided. The system includes an intake conduit of combustive air into the engine, a discharge conduit of the exhaust gases produced by the engine, and a cyclone separator associated to the intake conduit and provided with a collecting tank of detritus, a link conduit which has a greater diameter than the discharge conduit and which is inserted on at least a terminal tract of the discharge conduit, such that a hollow space is defined between the terminal tract and the link conduit, and a connecting conduit destined to connect the collecting tank with the hollow space by flowing into the link conduit upstream of an end of the discharge conduit with respect to an outflow of the exhaust gases.