Carburetor arrangement for changing the ratio of the air/fuel mixture in handheld motor-driven apparatus

A carburetor arrangement for an internal combustion engine of a handheld portable tool such as a chain saw, hedge trimmer and the like, the carburetor arrangement comprising:

• a carburetor housing having an interior space;
• a movable regulating membrane mounted in said space so as to partition the same into a first chamber on one side of said membrane and into a second chamber on the other side of said membrane;
• a fuel channel communicating with said first chamber and defining a valve seat;
• a valve needle mounted in said first chamber to define a valve with said valve seat;
• resilient means for resiliently biasing said valve needle into engagement with said valve seat;
• said valve needle being operatively connected to said regulating membrane so as to be displaceable against the force of said resilient means and away from said valve seat in response to a movement of said membrane thereby admitting fuel into said first chamber;
• actuating means operatively connected to said valve needle for additionally displacing the latter in the opening direction to open the valve a further incremental amount in response to a predetermined rotational speed of the engine thereby enriching the air/fuel mixture by an additional amount;
• a control membrane mounted in said interior space so as to be disposed adjacent the other side of said regulating membrane to define a wall of said second chamber and said control membrane having a first side facing said other side of said regulating membrane and a second side facing away from said regulating membrane;
• second resilient means for resiliently biasing said control membrane on said first side thereof in a direction away from said regulating membrane;
• passage means for conducting ambient air into said second chamber for charging said control membrane on said first side thereof;
• a pin mounted on said control membrane so as to be directed toward said regulating membrane;
• and,
• overpressure means for applying an overpressure dependent upon said rotational speed to said second side of said control membrane for displacing said control membrane against the force of said second resilient means and said pin toward said regulating membrane which, in turn, is displaced to act upon said valve needle to open said valve said incremental amount.