Subaru ff-1/1300/1400/1600/1800/Brat 1970-1984 Repair Guide

Engine Modification System



The operating principal of this system is not only to obtain correct air/fuel mixture while the vehicle is decelerating, but also to promote complete combustion by advancing or retarding the ignition timing, thus reducing the amount of emissions released into the atmosphere.

This system is found, with variations, on almost all models from 1972.

While the vehicle is decelerating, the primary throttle valve is closed, causing a high vacuum condition to occur in the intake manifold. This vacuum is conducted through a vacuum control valve and on to the carburetor where a by-pass jet is opened and extra mixture is allowed to enter the venturi below the throttle plates. This enriches the mixture and promotes cleaner combustion.

The vacuum is also routed to the distributor vacuum control. After passing through an air damper (1972-73 only) which regulates the vacuum to a smooth application, the distributor vacuum control advances or retards the ignition spark in order to promote complete combustion in the cylinders.

While the primary throttle plate is opened during acceleration, cruising or idling, the by-pass air valve in the carburetor does not open because the vacuum does not reach the specified measure. However, the distributor vacuum control operates at a much lower vacuum condition. Thus it is operating more often than the by-pass valve in the carburetor.

There is an anti-dieseling solenoid switch mounted on the side opposite the float bowl on the carburetor. The purpose of this switch is to prevent the engine from dieseling when the ignition switch is turned OFF . When the ignition switch is turned off, the electrical current which supplies an electromagnet in the switch is also cut off. A spring inside of the housing forces a plunger into position, blocking the fuel passages leading to the opening below the throttle plates. When the ignition switch is turned ON , it energizes the electromagnet in the switch and pulls the plunger out of the fuel passage, thus allowing fuel to reach the opening below the throttle plates.

On some models there is a solenoid valve located on the line to the distributor vacuum control. It is an electrically operated switch which receives a signal from the coolant temperature switch. At coolant temperatures below 100°F (38°C), the solenoid valve prevents engine vacuum from actuating the distributor retard mechanism. This allows for improved engine starting in cold weather.

On 1982-84 models, this system operates the vacuum advance diaphragm of the distributor via a thermal vacuum valve. The valve opens the entire vacuum advance circuit to atmospheric pressure in a certain temperature range. This range (measured at the intake manifold cooling water circuit) is 59°F (15°C) to 95°F (35°C) on all but the turbocharged engine. On the 1800 Turbo, this range is 113°F (45°C) to 131°F (55°C).

The valve simultaneously activates the EGR valve so that when vacuum advance is turned off the EGR valve does not work either. If this system is operating improperly, the car would exhibit very poor operation when cold. Symptoms such as very poor fuel mileage and performance with the engine hot, or slow warm-up may also occur.

Click image to see an enlarged view

Fig. Fig. 1 Engine modification system component locations

The system draws its vacuum supply from a port in the carburetor (or throttle body) above the throttle, so vacuum advance is not present at normal hot engine idle speed, but begins as the throttle is opened past idle. On certain models, advance is also desirable at idle. These incorporate a port located under the lower edge of the throttle plate which becomes ineffective above idle throttle opening. A check valve connects this port to the rest of the system. These models are: 1800 Turbo, 1982 1600 2-door Hatchback GL (5MT), 4-door GL (5MT) and Hardtop GLF (5MT), and all 1983-84 1600cc models. If the check valve fails, symptoms would include slow or erratic idle and, possibly, a slight hesitation.


Thermal Vacuum Valve (TVV)

To check the function of the thermal vacuum valve:

  1. Drain a little coolant and remove the air cleaner.
  3. Disconnect the hoses and remove the valve from the intake manifold.
  5. Cap off the center (EGR) port and connect hoses to the upper and lower ports. These hoses must seal tightly so no water will get into the top part of the valve. The tops of the hoses must also stay dry.

When draining the coolant, keep in mind that cats and dogs are attracted by ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Immerse the valve in a pan of water that also contains a 200°F (93°C) thermometer. If necessary, use ice to cool the water below 59°F (15°C) unless your car is an 1800 Turbo.
  3. Heat the water while blowing into the air cleaner hose connection.
  5. The valve should seal at first. As the temperature passes the 59°F (15°C) mark, or as it passes the 113°F (45°C) mark on the Turbo, the valve should open and you should be able to blow air through it freely.
  7. Again, at 104°F (40°C), 131°F (55°C) on the Turbo, the valve should seal tightly.
  9. If it fails any of these tests, replace the valve.
  11. Coat the valve with sealer before screwing it back into the intake manifold.
  13. To check for proper functioning of the check valve, install a timing light and idle the engine after it is fully warmed up.
  15. Accelerate the engine up to about 2000 rpm and then slowly return the throttle to normal idle position while watching the point at which the ignition fires.
  17. The timing should remain fully advanced right down to idle speed. If the check valve is stuck shut, timing will retard fairly suddenly at idle speed.
  19. If the check valve is stuck open, the timing will abruptly retard slightly above idle speed.
  21. If the valve fails these tests, note which end of the valve is connected to vacuum lines leading to the distributor, and then pull off the vacuum lines on either end of the valve.
  23. To confirm your test results, blow through the valve in the direction of flow from the carburetor lower port toward the distributor. Air should flow freely.
  25. Then, turn the valve around and blow through it in the other direction. The valve should seal tightly.
  27. Replace the valve if either test is failed.

Click image to see an enlarged view

Fig. Fig. 2 The ignition control system for the 1982-84 models have two versions, pictured here. See text for exact model delineation


Remove the spare tire and the air cleaner assembly from the engine compartment to gain access to the various components of the engine modifications system.

Vacuum Control Valve
  1. Detach the vacuum hoses from the distributor retard unit, carburetor by-pass servo diaphragm, automatic choke main diaphragm (if so equipped), and the intake manifold.
  3. Remove the bolts which secure the vacuum control valve to the intake manifold.
  5. Lift the valve assembly off the manifold. On 1972-73 models, the air damper (accumulator) is connected to the valve bracket and can be removed at the same time.
  7. When installing the valve, be sure to route the vacuum hoses correctly.

Click image to see an enlarged view

Fig. Fig. 3 Cutaway view of vacuum control valve

Anti-Dieseling Solenoid
1972-73 MODELS
  1. Disconnect the lead which runs to the anti-dieseling solenoid.
  3. Remove the 3 screws which secure the solenoid to the carburetor.
  5. Carefully remove the solenoid assembly from the carburetor.

1974-82 MODELS
  1. Disconnect the solenoid electrical lead.
  3. Unscrew the solenoid assembly.
  5. Remove it, complete with gasket from the carburetor body.
  7. Be sure to install the gasket before installing the solenoid assembly.

Carburetor By-pass Servo Diaphragm

Be careful not to confuse the servo diaphragm with the choke diaphragm used on 1974 and later carburetors.

  1. Remove the vacuum hose from the servo diaphragm.
  3. Remove the 3 screws that attach the diaphragm to the carburetor.
  5. Remove the servo diaphragm.
  7. Installation is performed in the reverse order of removal.