PURPOSE: To compensate a low speed system and the main fuel system, by arranging in addition to a main controlling electromagnetic valve an auxiliary electromagnetic valve in parralel in a main fuel passage of a carbureter, and allowing the auxiliary electromagnetic valve to be actuated when high output state required.

CONSTITUTION: A main electromagnetic valve 19 and an auxiliary electromagnetic valve 20 are arranged in a float chamber 5. Bleeders 24 control the quantities of air passing through air bleeders 15, 16, that is, the quantities of air to an air passage connected to a low speed fuel passage 12. By a signal outputted from a highly compensating unit 30 the auxiliary electromagnetic valve 20 is acutated to compensate the denseness due to the change of air density.