PURPOSE: To obtain stable operation from changing a carburetor fuel level according to atmospheric temperature by a method wherein a bimetal member is formed in circular are or semicircular arc with a high expansion material inside and is provided in a float valve working zone of a carburetor float arm.

CONSTITUTION: To obtain a change in a fuel level having a satisfactory strength structurally and necessary in the working temperature range of an engine, a bimetal member is used in a working zone 2 of a float valve of a float arm 3 to push up a float valve 1. To utilize a curvature of the bimetal effectively in this case for displacement of the float valve 1, the bimetal is preferred to have a shape of semicircular arc or circular arc as illustrated in (a)W(c), a contact face with the float valve 1 is also made in circular arc, thereby keeping the float valve from side pressure at the time of bimetal displacement. Operation stable through all seasons is thus obtainable.