In this paper the authors present a variational finite element principle tocompute the fuel supply characteristics in the carburettor. The principle is employed to obtain the vacuum performance at the fuel outlet and the fuel supply characteristics of the main fuel system of the carburettor in a condition of opening values with distinguishing performance of the vacuum along the inside wall of the carburettor. Details of the results obtained in the corresponding test are also reported in this paper. Good agreement is found between the computational results and those of the test.Compared with 1-D computation, 2-D computation is not only good for saving the fuel, but also good for improving the aerodynamic performance of the carburettor. It provides a new approach for the design of the carburettor.