I ahve a 73 mustamg i recovered out of a paddock, there is two ports on my modulator valve (fmx tramnsmission) can anyone identify are the both vacuum or not
One port in mioddle at back and one port on side facing drivers door, the car had not a single wire or vacuum hoses so i need a diagram or advice please

That particular modulator is know as a altitude compensating style.

The large hole is for shifting purposes.

The small one is to compensate for variances in altitude and not really necessary.

You can put a plug over it, I have plugged oodles and had no problems.
If the chamber is left open dirt and moisture can enter.
Dirt will eventually contaminate the valve while moisture can freeze and upshifting will be very delayed or nonexistant until the ice melts.


This is an exhaust gas recirculation device for high altitude use. The engine comprises an exhaust gas recirculation control valve device communicating an intake manifold with an exhaust manifold for circulating a part of the exhaust gas for reducing harmful NOx contaminants contained in the exhaust gas, which device is actuated by vacuum supplied from a vacuum port disposed at a position adjacent to a throttle valve of a carburetor, a reservoir tank for storing a vacuum; a control valve device for controlling the delivery of the vacuum from the reservoir tank; and an altitude compensating device for actuating the control valve device in accordance with the altitude.
Utilizing the above-described device, exhaust gas is recirculated for a predetermined interval of time after the throttle valve has been opened at a level which is more than the predetermined level during high altitudes. However, exhaust gas is stopped from being recirculated as soon as the throttle valve is opened to the predetermined level during low altitudes. As a result, the amount of harmful NOx contaminants contained in the exhaust gas can always be maintained at a low level.

The large hose in the center is for manifold vacuum which controls the shift and is connected to a manifold vacuum source. The small hose on the side leads to a seperate vacuum chamber inside the modulator and is connected to carb or EGR vacuum . The porpose of this chamber is to modify the effective area of the manifold vacuum chamber and compensates for EGR valve action. You dont necessarlly need the small hose but dont plug off the connector. It needs to breath or it will effect the function of the modulator.
Mac

On modulators manufactured by the VXXXXXX Co. the compensator vacuum connector was a phoney which went nowhere so pluging it off meant nothing. OEM modulators manufactured by Schovile. Century Brass and Harrison actually had a compensateing diaghram and lab test show that pluging off the port effects the value and response of the main diaghram as much as 5 0/0. If there is a concern about contamination or moisture entering then cut off a small piece of a breather element filter and shove it into the port.
Mac

Thank you all very much, I've hooked up centre to vacumm and capped off the side port, now I just need to rewure the engine bay nad I'll test it out

Iguess we are all entitled to a few mistakes. You just changed a 12lb modulator to a 14 lb'er and delayed diaghram action by 5 to 10 0/0. Why don't you just use a single hose modulator and eliminate the problem.
Mac