I have an '81 F-100 with a 302 , auto , A/C , P/B , Federal emissions . It is missing a vacuum control shown on the emissions sticker as a VRV , which seems to translate as vacuum regulator valve . It shows ported and manifold vacuum coming into one side with one line out to the distributor . What does it do , how important ? The catalytic converter and most of the air injection was gone when I got the truck many years ago but the other devices are there and working even though I am converting to a four barrel carb . There are no inspections in this area . Thanks .

 

From Ford: "The three-port and four port regulators are used to control the vacuum advance to the distributor. During engine idle conditions, the manifold vacuum signal is reduced to a constant output signal. Off idle, the output signal equals the spark port." So without knowing what distributor (DS-II?) and carb (Ford 2150-2v was stock?) you are using, hard to say wether or not it is important. The VRV came in three different colors with three different levels of vacuum. Hope this helps.

 

All stock for the year - DS-II distributor with one port on vacuum advance , 2 bbl carb , I presume 2150 - not variable venturi [I wouldn't have bought one with one of those] . The emissions sticker showed 3 ports on the VRV , no idea what color . I can post the calibration code if that is useful [tomorrow-too many skeeters out tonight] . Do I deduce that the VRV device supplied a low level of vacuum at idle then rose to ported level off idle , did it also raise the vacuum signal under the high vacuum of deacceleration ?

 

If half your emissions are gone, and then you are going to change the carb, just run a single line from the ported vacuum on the new carb to the distributor. Skip all that other stuff.

 

My conversion will have EGR , EVAP [incl. carb vent] , thermostatic air cleaner and the AIR converted to a pulse system . The only component completely missing was the cat [catalysts and carbs were not a good combination-traded carbon monoxide for sulfur dioxide] . Is the effect of the VRV minor in the overall operation of the engine ? I don't want to pollute the air anymore than I can avoid , but I'm not going to spend $$$ for little effect on a truck that gets driven less than 5000 miles a year either .

 

On some models, the distributor vacuum line connects to the center port of the VRV that is screwed into the coolant elbow. From the VRV, one vacuum line goes to ported vacuum, and the other line goes to full manifold vacuum. At normal operating temperature, the distributor uses ported vacuum for ignition timing. This particular VRV is rated at 225 degrees, which means when the coolant reaches that temperature, the VRV turns off ported vacuum and then uses full manifold vacuum for ignition timing. This happens because at 225 degrees, the engine is beginning to overheat, and full manifold vacuum will raise your idle speed and also run the water pump at a faster speed. This is to help cool the engine back down. When the coolant temperature is under 225 degrees again, the VRV will turn off full manifold vacuum and return to ported vacuum.

 

The schematic on my truck shows the device as not attached to a coolant passage with the vacuum sources shown as coming directly from the manifold and carburetor [a ported vacuum source] and then directly to the carb . It begins to seem like this device would have little effect either on emissions or driveability . I also have not found it available at aftermarket parts sources and probably wouldn't want to pay dealer price . The engine has run well with it missing , even with leaky valves . I gathered the carb and manifold in preparation for doing the valves and , on reassembly , found this was missing as I figured out how to reroute the vaccum lines to the relocated temperature sensing VCV . I believe I will quit worrying about it . Thanks , all , for the input .