A cam, connected to the primary throttle butterfly spindle by a « Flector » acts, via a roller, on a lever which tensions a spring which in turn permanently applies pressure to a slide valve. Another « By-pass » slide-valve is pushed towards the first by a week spring. In the of this slide-valve the diameter is smaller than the bore in which it operates. 3. Principle and Operation : a) De-clutching : Principle :