Vacuum operable units for use with ignition distributors

Claim 1

An ignition timing control system for use in an ignition distributor for a road vehicle said distributor including a casing, a shaft journaled for rotation within said casing and including a cam, a plate movable angularly within the casing relative to said shaft, a contact breaker assembly carried by said plate and operable in response to rotation of said shaft and a vacuum operable unit carried by said casing, said unit comprising a casing, a diaphragm dividing said unit casing into first and second chambers, a plunger slidably received in the first chamber, the plunger being secured at one end to the diaphragm and extending at its other end from the first chamber, first and second conduits communicating with the first and second chambers respectively, first resilient means within the first chamber anD acting on the diaphragm to resist movement of the diaphragm in a direction to reduce the volume of the first chamber, second resilient means in the second chamber, and acting on the diaphragm in a direction to resist movement of the diaphragm in a direction to reduce the volume of the second chamber, the first and second resilient means acting on the diaphragm so that the diaphragm is urged by the first and second resilient means to a rest position corresponding to the normal zero advance position of the distributor, stop means associated with the second resilient means in the second chamber, the stop means preventing said second resilient means acting on the diaphragm during reduction in volume in the first chamber, and sealing means acting between the wall of the first chamber and the plunger to seal the first chamber, the sealing means allowing relative movement between the plunger and the wall of the first chamber, and including first and second annular members secured to the plunger and to the wall of the first chamber respectively, and a flexible tubular member interconnecting the first and second annular members, the diameter of the tubular member tapering from a maximum adjacent one of the annular members to a minimum adjacent the other of the annular members, and the tubular member being turned inside out intermediate its ends so that part of the tubular member surrounds a further part thereof and the tubular member together with the first and second annular members define a rolling seal, the plunger of said unit being connected to said plate so that axial movement of the plunger varies the ignition timing of the distributor, and a valve operable in a first position, during normal running of the engine of the vehicle, to connect one of said chambers to atmosphere while connecting the other chamber to the air inlet side of a throttle valve of the engine, so that variations in pressure at the air inlet side of the throttle valve serve through the unit to control the ignition timing of the distributor to advance the ignition timing of the distributor from, or retard the ignition timing of the distributor to, said normal zero advance position and operable in a second position, during idling or overrun conditions of the engine, to connect said one chamber to an inlet manifold of the engine while connecting the other chamber to atmosphere so that the reduced pressure in the inlet manifold serves through said unit, to retard the ignition timing of the distributor from said normal zero advance position.