PURPOSE: To supply air-fuel-mixture having proper concentration according to operating condition of an engine at all times so as to improve accelerating performance and fuel consumption by varying the capacity of a well provided in the main system of a carburetor.

CONSTITUTION: In a carburetor, a main system 22 is provided in a float chamber 14, and it is composed of a cylindrical needle jet 23, a well 24 formed around it, a main jet 25, and an air jet 26. An auxiliary well part 28 is opened in a part of the well 24 side in this main system 22, and the plunger 29 of a solenoid 30, which is a means for varying the capacity of the well 24, is fit to the inside of the auxiliary well part 28. At the time of cooling engine, the plunger 29 is stored in the solenoid 30 by operation of a temperature sensing switch 35 so as to enlarge the capacity of the well 24. While after warming-up of engine is completed, the plunger 29 is projected outside the solenoid 30 so as to reduce the capacity of the well 24.