Carbureted Models Only
See Figures 1 and 2
On carbureted Toyotas with an engine modification system, a throttle positioner is included to reduce exhaust emissions during deceleration. The positioner prevents the throttle from closing completely. Vacuum is reduced under the throttle valve which, in turn, acts on the retard chamber of the distributor vacuum unit. This compensates for the loss of engine braking caused by the partially opened throttle.
For a description of the operation of the dual-diaphragm distributor, see Dual-Diaphragm Distributor.
Once the vehicle drops below a predetermined speed, the vacuum switching valve provides vacuum to the throttle positioner diaphragm; the throttle positioner retracts allowing the throttle valve to close completely. The distributor also is returned to normal operation.
- Start the engine and allow it to reach normal operating temperature.
- Adjust the idle speed.
Leave the tachometer connected after completing the idle adjustments, as it will be needed in Step 5.
- Detach the vacuum line from the positioner diaphragm unit and plug the line up.
- Accelerate the engine slightly to set the throttle positioner in place.
- Check the engine speed with a tachometer when the throttle positioner is set.
- If necessary, adjust the engine speed, with the throttle positioner adjusting screw.
- Connect the vacuum hose to the positioner diaphragm.
- The throttle lever should be freed from the positioner as soon as the vacuum hose is connected. Engine idle should return to normal.
- If the throttle positioner fails to function properly, check its linkage, and vacuum diaphragm. If there are no defects in either of these, the fault probably lies in the vacuum switching valve or the speed marker unit.
Due to the complexity of these two components, and also because they require special test equipment, their service is best left to an authorized facility.