The induction tube of an internal combustion engine is provided with an air flow measuring device which may be part of a carburetor and with a normal throttle valve, settable arbitrarily from the outside. A bypass channel conducts air around one or both of these elements and contains a flow restricting device. The induction tube further includes an air flow throttling mechanism for establishing a definite pressure drop across the inlet and terminus of the bypass to insure air flow therethrough even at full throttle and low rpm when the engine vacuum is low. The air flow throttling mechanism may be a pivoted baffle plate or a slide which enters the induction tube and partially blocks the air flow therethrough and which is moved by differential pressures in the induction tube. The slide retracts from the induction tube at full throttle and at high engine speed.