Bob K -- based upon the schematics of the 1988 YJ 258 BBD feedback carb, the vacuum hose feeding to the vac adv of distributor receives/ carries manifold vacuum , not ported vac . The manifold vacuum source can be any inlet at the base of the Carter BBD ; the base carb inlets are all manifold vacuum sources. If my memory serves me correctly , the one that is OEM setup comes off the brass fitting near the EGR . The 2 fittings bolted to the firewall, called the " Vacuum Switch Assembly " , are the "green 10 inch manifold vac switch" and the "white 4 inch ported vac switch" . They switch " open / closed" and can feed juice back to the MCU based upon engine load : that is based upon the change in manifold vac and ported vac. The switches of the 10/ 4 are controlled by separate vacuum hoses feeding into each of them separately . There is a black /purple delay valve coupled with a small black vacuum reservoir built in-line with one of them , that is into the " green 10 inch manifold vac switch " feed hose . The MCU receives the input from the Vacuum Switch Assembly and then does its thing . I have been unable to read a detailed explanation of exactly what the MCU will do in response to the input from the 10/4 switches . I am theorizing that either the MCU changes timing / the MCU changes air fuel mix . This has stumped the Jeep dealer mechanics . The only source of ported vac would be from the location above the throttle plates , the carb outlet half way up the carb and facing the valve cover . This ported vac source also feeds ported vac to the 4 inch vac switch on the firewall , feeds ported vac to the CTO --> TVS in air box --> EGR system , feeds the ported vac to CTO --> Charcoal Evap Canister "signal" purge inlet . Hope this applies to your rig. I do not know if you rig is same as mine .