Vacuum operable units for ignition distributors

[1. A vacuum operable unit for use with an internal combustion engine ignition distributor including a hollow casing having a pair of fixed outer walls and divided into first and second chambers by a flexible diaphragm, a push rod coupled at one end to the diaphragm so as to move therewith and arranged at its other end to be coupled to the timing plate of an ignition distributor, said rod extending through said outer wall of said first chamber and sealing means engaging said outer wall of the first chamber and the rod to seal said first chamber while permitting movement of the rod, first resilient means within the first chamber having first retaining means at a first end thereof which is operatively acted upon by the diaphragm during movement of the diaphragm in a first direction from a central position thereof to reduce the volume of the first chamber, a second end thereof restrained by said outer fixed wall of said first chamber, second resilient means within the second chamber having second retaining means at a first end thereof which is operatively acted upon by the diaphragm during movement of the diaphragm in a second direction from said central position to reduce the volume of the second chamber, a second end thereof restrained by said outer fixed wall of said second chamber, and first and second abutment means fixed to the casing and engagable by said first and second retaining means respectively in said central position of the diaphragm for preventing said first and second resilient means respectively from operatively acting on said diaphragm in said central position, and first and second extensions on said first and second retaining means, respectively, extending through and beyond said first and

• second abutment means, respectively, for engaging said diaphragm.] 2. [A unit as claimed in claim 1 wherein] A vacuum operable unit for use with an internal combustion engine ignition distributor including a hollow casing having a pair of fixed outer walls and divided into first and second chambers by a flexible diaphragm, a push rod coupled at one end to the diaphragm so as to move therewith and arranged at its other end to be coupled to the timing plate of an ignition distributor, said rod extending through said outer wall of said first chamber and sealing means engaging said outer wall of the first chamber and the rod to seal said first chamber while permitting movement of the rod, first resilient means within the first chamber having first retaining means at a first end thereof which is operatively acted upon by the diaphragm during movement of the diaphragm in a first direction from a central position thereof to reduce the volume of the first chamber, a second end thereof restrained by said outer fixed wall of said first chamber, second resilient means within the second chamber having second retaining means at a first end thereof which is operatively acted upon by the diaphragm during movement of the diaphragm in a second direction from said central position to reduce the volume of the second chamber, a second end thereof restrained by said outer fixed wall of said second chamber, and first and second abutment means fixed to the casing and engageable by said first and second retaining means respectively in said central position of the diaphragm for preventing said first and second resilient means respectively from operatively acting on said diaphragm in said central position, and first and second extensions on said first and second retaining means, respectively, extending through and beyond said first and second abutment means, respectively, for engaging said diaphragm, one of said first and second retaining means includes a means for adjusting its respective extension through which that retaining means engages the diaphragm, said extension being adjusted to ensure that the diaphragm does not have any free movement relative to said first and second resilient means while at the same time ensuring that in said central position
• neither resilient means acts on the diaphragm. 3. A unit as claimed in claim 2 wherein said means for adjusting its respective extension is adjustable from the exterior of the unit by way of a sealable aperture in
• casing. 4. A unit as claimed in claim 2 wherein said means for adjusting its respective extension is a screw in screw threaded engagement with a component of said one retaining means, said component including means for holding it against rotation relative to the casing to facilitate adjustment of the position of the screw.