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PECV [Power Enrichment Control Valve]
This is one of those parts that are unobtainium.
This valve is used to control the Power Piston in the carburetor, to increase the fuel delivery during boost conditions, and prevent boost pressure from reaching the power valve. It is screwed into the top/front of the manifold where it senses the vac./boost pressure. There are three other vac. lines connected to it. The centre port is connected to the carburetor power piston and controls the richness of the mixture. The other two ports are connected to the carburetor vac. source and the vent of the carburetor.
Below the valve threshold, the carburetor power piston is connected to the carburetor vac. source and is controlled according to the engine load. As the pressure increases to a point above the valve threshold the carburetor power piston is connected to the carburetor vent. This allows the power piston spring to lift the power piston fully and the carburetor goes to full rich.
When the diaphragm in this valve fails, the carburetor power piston will most probably see positive pressure under boost, leading to a lean condition. There are then two choices, connect the carburetor for a full rich, or full lean condition. Neither of these choices is good. Under full rich conditions the fuel consumption will be atrocious, and excess fuel will wash the oil film from cylinder walls causing premature wear. The full lean condition will cause detonation on boost. Either of these conditions can result in severe engine damage.
PECV ALTERNATIVE
This is the simplest option I have found and will give better results than the full lean or full rich conditions. It only requires obtaining one part from your local parts store or wrecking yard, and a simple bit of wiring. The part required is a "three port vacuum switch". I obtained mine at the wrecking yard from some unidentified Japanese car. Any three port switch will work.
To get power to energise the switch, connect it to the yellow boost indicator light. I found on my car that the yellow light comes on at zero boost, and the red light comes on at about 5 P.S.I.. On the firewall there are two vacuum switches which operate the boost indicator lights,one for the red and one for the yellow light. Check the switch that operates the yellow light for the ungrounded side. This is where one side of the three port vacuum switch should be connected. The other side of three port vac. switch should be connected to a 12 volt switched source. With this wiring the switch will change the power valve from vacuum control, to full rich, at zero P.S.I..
The original PECV should be removed from the manifold and the hole plugged. Three vacuum connections are required. The common port of the vac. switch should be connected to the power valve port on the carburetor. The normally open port should be connected to the manifold vac./boost port, and the normally closed port to the carburetor vent port. It will be necessary to use some "T" connections to accomplish this, as you do not want to disconnect any other original vacuum connections.
INSTALLATION
The installation is simple and only requires finding a place to mount the three port vac.switch, and then piping it into the system as shown. The switch should be mounted as close to the boost switches as possible to avoid long vac. lines. Only two wires are required, one from the boost light to the vac. switch and one from the vac. switch to a 12 volt switched source, as shown in the wiring diagram. Under the dash there is convenient grommet where the wires can be passed through from the cockpit to the engine compartment. The grommet is located just above and to the right of the steering column.
If the vac. switch ports are not identified, mine were not, they can be identified in the following manner.
Blow through the ports. The two that you can blow through are the common and normally open. Now energise the switch and blow through again . The ports you are now blowing through will be the common and the normally closed. The common port will be that which you can blow through under both conditions.
TEST AND ADJUSTMENT
To test the function just drive the car and listen for a click as the yellow light comes on. This is the vac.switch switching, at which point the carb. goes to full rich. There are no adjustments to be made because the whole process is controlled by the boost light switch.
