main barrel 18 and a secondary barrel 19 which serve respective adjacent intake passages 15 the carburetor barrels 18 and 19 are served by a common float chamber 21 and accelerating pump 22 a main throttle valve 23 is positioned in the main carburetor barrel 18 downstream of its respective venturi section 24 the main throttle valve 23 is supported in the carburetor body upon a throttle valve shaft 25 the accelerating pump 22 includes an accelerating pump lever 26 that is actuated by means of a link 27 that is connected to a lever 28 affixed to the throttle valve shaft 25 in a known manner fuel is supplied to the float chamber 21 in a known manner by means including an inlet fitting 29 the carburetor barrel 18 and only this barrel of the carburetor 17 is provided with an idle and low speed fuel discharge circuit indicated generally by the reference numeral 31 having an adjusting needle 32 idle and low speed fuel requirements for the two cylinders served by the carburetor 17 is all supplied by the carburetor barrel 18 via a supplemental or sub intake system now to be described the sub intake system includes an inlet passage 33 formed in the carburetor barrel 18 downstream of the main throttle valve 23 and upstream of a sub throttle valve 34 that is also positioned in this carburetor barrel a spacer plate 35 which is positioned between the carburetor 17 and the cylinder head 13.