PURPOSE: To reduce dimensions of a member associating with the atmosphere, by providing to a release passage communicating the working pressure chamber of hydraulic timer with a fuel sump a variable orifice associating with the atmosphere.

CONSTITUTION: A timer piston 28 has its working pressure chamber connected with a fuel tank 4 via a release passage 33. A piston 38 forming a variable orifice is interposed intermediately at the release passage 33, the piston 38 being axially displaced through its association with the atmosphere as caused by a bellows 40.