v grooves 22 23 there is a cylindrical surface 29 on the housing 10 for cooperation with a seal element 30 carried by the handle 13 to seal off an exhaust air communication opening 31 in the housing 10 and the exhaust passage 17 in the handle 13 from the atmosphere at its inner end the handle 13 is provided with an enlarged opening 32 which is surrounded by the seal element 30 and which is large enough to encircle fully the opening 31 in the housing 10 no matter what the angular position of the handle 13 is the opening 31 in the housing 10 as well as the opening 32 in the handle 13 and the seal element 30 are of rectangular shape as illustrated in fig6 the smaller housing opening 31 is illustrated in dash lines as being apparent from fig1 5 the opening 32 at the inner end of the exhaust handle 13 is offset some 10 u00b0 from the centre line of the handle due to this and due to the fact that the handle is symmetric about the central section plane shown in fig5 handle 13 could be mounted upside down or in two alternative 180 u00b0 turned over positions thereby the angular adjustability of the exhaust handle is increased by approx 20 u00b0 in the illustration in fig1 this feature has been used to obtain as wide an adjustment range as possible for the handle 13 also the mounting of the handle 12 is symmetric about a central horizontal plane which makes this handle too shiftable between two alternative positions in the position shown in fig1 and 2 the handle 12 is mounted with the throttle valve lever.