PURPOSE: To enhance controllability by providing a fuel and air supply part around a nozzle formed by a rotation-symmetrical nozzle body and throttle body, and providing regulation means, for shifting the throttle body in an opening direction, and spring elements for returning the throttle body.

CONSTITUTION: In a nozzle body 2 having an approximately rotation-symmetrical interior wall 3 which tapers upward from a lower range 4 to the narrowest inner cross-sectional part 5, a rotation-symmetrical conical throttle body 7 is provided in such a way as to be freely axially moved for adjustments. The nozzle body 2 is divided into upper and lower portions within the range of the narrowest cross section 5 and an annular air-fuel gap 11 is formed between the upper and lower nozzle body portions 2a, 2b and around a nozzle 10. In order to guide the throttle body 7, a guide pipe 18 placed coaxially with respect to the vertical axis 1 of the nozzle body 2 is secured to a casing part 34 and the throttle body 7 can be shifted via a rod 20 passed through the guide tube 18 and via a lever 21. The throttle body 7 is energized by compression springs 29a, 29b in its closing direction.