referring to the drawing , the novel valve constructed according to the invention and shown in fig2 has a body 1 enclosing a cylindrical chamber 2 and comprised of a cup - like outer cover 3 assembled on a cup - like inner cover 5 with an &# 34 ; o &# 34 ; ring 7 therebetween . outer cover 3 has a port 9 and inner cover 5 has a port 11 connected to chamber 2 . an air filter 13 of paper or other suitable material is retained by an annular retainer 15 in a cylindrical recess 17 in outer cover 3 connected to port 9 . a filter 17 of paper or other suitable material is retained by an annular washer 19 in a cylindrical recess 21 in inner cover 5 connected to port 11 . filters 13 and 17 filter air as it passes through chamber 2 . inner cover 5 has an annular groove 23 formed therein for receiving an &# 34 ; o &# 34 ; ring 25 . a disk 27 is urged into engagement with &# 34 ; o &# 34 ; ring 25 by a coil spring 29 having one end seated on the disk and the other end seated on a shoulder 31 in a cylindrical cavity 33 in outer cover 3 . disk 27 has a calibrated orifice 35 therethrough for restricting air flow to permit rate limited equalizaton of pressure between ports 9 and 11 . the tension of spring 29 is such that when the pressure at port 11 is greater by a predetermined amount than at port 9 disk 27 is lifted off of &# 34 ; o &# 34 ; ring 25 in opposition to the force of spring 29 and air flow from port 11 to port 9 is unrestricted . in one embodiment the pressure differential is 2 . 5 inches hg . an umbrella valve 37 of rubber or other suitable material is positioned in an aperture 39 substantially centerally in disk 27 . the valve has a shoulder 41 engaging one side of the disk adjacent the aperture and an umbrella portion 43 seated on the other side of the disk when the valve is closed . vents 45 are provided in disk 27 adjacent shoulder 40 of umbrella valve 37 to permit air flow from port 9 to umbrella portion 43 of valve 37 . when the pressure at port 9 is slightly higher than the pressure at port 11 the umbrella portion of valve 37 is lifted from its seat and air flows unrestricted from port 9 through vents 45 to port 11 . when the valve is used to control the spark timing in an internal combustion engine on a vehicle as described above , port 11 is connected to carburetor spark port 47 and port 9 is connected to distributor vacuum advance chamber 49 as shown in fig1 . when the pressure at the carburetor spark port is less than a predetermined amount below the pressure at the distributor vacuum advance chamber disk 27 is seated on &# 34 ; o &# 34 ; ring 25 and air flow from port 11 to port 9 is restricted and spark retard is delayed . orifice 35 then permits rate limited equalization of pressure from the carburetor spark port to the distributor vacuum advance chamber . disk 27 is lifted off of &# 34 ; o &# 34 ; ring 25 when the pressure at port 11 is the predetermined amount or higher than at port 9 to permit unrestricted air flow from port 11 to port 9 . umbrella valve 37 is lifted off of its seat on disk 27 when the pressure at port 9 is only slightly higher than at port 11 to permit unrestricted air flow from port 9 to port 11 . with this arrangement when the differential pressure between the ports is the predetermined amount or greater , as in substantially full throttle acceleration , the spark retard operates in the normal manner , but when the pressure differential between the ports is less than the predetermined amount , as in moderate vehicle acceleration , spark retard is delayed . spark advance operates in the normal manner at all times . operating the engine in this manner reduces emissions and improves engine performance and economy .