PURPOSE: To allow air to flow smoothly with the provision of the shorter length of duct by integrating both a main passage which communicates an air cleaner with an intake port of a carburator, and a sub-passage which communicates the air cleaner with each of air jet passages, into one duct.

CONSTITUTION: A variable choke type carburator 11 is provided with a diaphragm chamber 22 above an intake passage 21 which has an inlet port 12 and an outlet port 20 at both sides of the passage, and with a combustion chamber 23 underneath the above said passage. The opening area of a main nozzle 30 which is communicated with the combustion chamber 23, is regulated by inserting a jet needle 29 located on a piston main body 27 which is installed on a diaphragm 24 in the diaphragm chamber 22. And a duct 35 is connected with the inlet port 12 of the carburator 11 so as to communicate the intake port with an air cleaner 41, and the duct 35 is formed integrally together with both a main passage 37 at the center, and sub-passages 38 and 39 which are communicated with each rear opening of an air intake port 15 and an air jet passage 14a, at the upper and lower sides of the passage 37.