wherein:

said air-fuel ratio sensor is mounted on an inner side wall portion of the exhaust pipe so as to be pointed upwardly toward a lateral center plane of said motorcycle, as viewed in front elevation of said motorcycle,

said inner side wall portion of the exhaust pipe faces towards the lateral center plane of said motorcycle; and a substantial portion of said air-fuel ratio sensor is located above a central horizontal plane bisecting a cross section of said exhaust pipe;

said air-fuel ratio sensor has a central axis which extends substantially radially with respect to a longitudinal axis of the exhaust pipe;

said air-fuel ratio sensor comprises a substantially cylindrical end cap, the end cap comprising a tip portion received within an interior of the exhaust pipe, and a base portion situated adjacent the tip portion,

said tip portion has a plurality of gas-receiving holes formed therein, said plurality of gas-receiving holes comprising a first group of gas-receiving holes arranged circumferentially around the tip portion, and a second group of gas-receiving holes arranged circumferentially around the tip portion, wherein the first group of said gas-receiving holes is axially spaced apart from the second group of said gas-receiving holes;

the exhaust pipe has an opening formed therein at said inner side wall portion thereof, and a mounting nut is fixedly attached to the exhaust pipe at said opening, wherein an axial length of the mounting nut is greater than twice the thickness of the exhaust pipe; and

wherein said base portion of the air-fuel ratio sensor has male threads formed thereon which are engaged with corresponding female threads formed on an inner surface of the mounting nut for operatively securing said air-fuel ratio sensor on said inner side wall portion of the exhaust pipe.