The ignition advance of an engine is controlled in response to engine speed and inlet manifold depression. The inlet manifold depression is sensed via two ducts 13, 21 each provided with a calibrated orifice 14, 22 and connected to the chamber 7 of a pneumatic actuating device 6. At low loads the throttle 2 occupies the position 2b between the two ducts so that a relatively reduced advance of the ignition is obtained by movement of the rod 5. At high loads, both ducts are downstream of the throttle and maximum advance is provided. IMAGE
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