Membrane fuel pump for a membrane carburetor

A membrane fuel pump for a membrane carburetor of an internal combustion engine including a two-stroke engine of a portable handheld work apparatus such as a motor-driven chain saw and the like, the carburetor having a pressure controller and the engine having a crankcase wherein pressure fluctuates during operation of the engine, the membrane fuel pump comprising:

• a housing defining an enclosed space;
• a pump membrane mounted in said housing and partitioning said space into a pump chamber and a drive chamber;
• a line connecting said crankcase to said drive chamber for charging said drive chamber with the pressure in said crankcase;
• fuel supply means for supplying fuel to said pump chamber;
• a first check valve interposed between said pump chamber and said fuel supply means;
• pressure connection means for connecting said pump chamber to the pressure controller of the membrane carburetor;
• a second check valve interposed between said pressure connection means and said pump chamber so as to permit fuel to flow from said pump chamber through said pressure connection means to said pressure controller of said carburetor when said second check valve is open and to interrupt the flow of fuel when said second check valve is closed;
• said housing including a bypass channel connecting said pressure connection means to said fuel supply means;
• and,
• valve means interposed between said pressure connection means and said bypass channel and being responsive to the pressure present in said pressure connection means for switching said valve means between a first position wherein said bypass channel is open to said pressure connection means and a second position wherein said bypass channel is closed to said pressure connection means.