SUBARU 1300 & 1400 Engine Modification: Operation

The engine modification system controls ignition timing advance and retard, controls air/fuel ratio during deceleration, and promotes afterburing of exhaust gases to prevent emissions.

1. When primary throttle valve of carburetor closes during deceleration, vacuum is created in intake manifold. A hose connecting manifold to vacuum control valve conducts vacuum to valve chamber "A". This causes a diaphragm in valve to open a poppet valve.
2. Vacuum is then conducted to servo diaphragm on carburetor by-pass system. This causes by-pass valve to open, connecting air horn of carburetor to lower portion of secondary throttle valve.
   Fig 1: Vacuum Control Valve

   Courtesy of SUBARU OF AMERICA, INC.
3. After by-pass valve is opened, air is conducted to by-pass valve from air horn. Air and fuel are metered at each by-pass jet and by-pass air bleed. Calibrations insure that proper air/fuel ratio is delivered to combustion chambers.
4. Ignition timing retard is brought about during deceleration by means of vacuum created in intake manifold. A vacuum line connecting manifold to air damper and then to distributor retard diaphragm conducts vacuum to distributor to promote afterburning.
5. During normal operation (acceleration, cruising and idling), a vacuum of 7.9 in. Hg is required to retard ignition timing, and 18.7 in. Hg is required to operate by-pass valve
6. In addition to vacuum operated modification system, an electrically controlled fuel shut-off is provided. A solenoid switch is installed in primary slow passage of carburetor and operated through ignition/starter switch. System control dieseling by cutting off fuel of slow system when ignition is shutoff.

Fig 2: Solenoid Switch (Fuel Cutoff)

Courtesy of SUBARU OF AMERICA, INC.