A carburetor for an internal combustion engine includes a power boost device for boosting engine output during acceleration or at high speeds which adjusts its air-fuel ratio and/or boost timing according to the alcohol content of the fuel. The power device includes a nitrile rubber swelling member which swells in accordance with fuel alcohol content and which is used to adjust the fuel flow aperture of a power jet. In addition, a second swelling member can be used as a linkage between an intake manifold-vacuum-responsive piston and a metering valve to adjust the timing or threshold vacuum level at which the metering valve begins to operate.