To provide a carburetor for supplying dense fuel and air-fuel mixture to an engine to facilitate the starting of the engine.

This carburetor has a fuel regulating rate diaphragm which forms an air chamber and a fuel chamber on one side and the other side, respectively. The carburetor is provided with a first flow passage leading to an air chamber and leading to a crankcase chamber of the engine, a second flow passage formed within the body and a first concentration valve leading to the first flow passage. The first concentration valve is movable between a first position for allowing fluid to flow from the first flow passage to the air chamber and a second position for preventing fluid from flowing from the first flow passage to the air chamber. A second concentration valve leading to the first flow passage is provided to affect flow of fluid passing through the second flow passage and to selectively enforce concentration of fuel and air-fuel mixture fed to the engine.