Found the cause of the paradox of rich mixture and low vacuum on my
2.8L. Eventually I located a teeny pin hole in the deceleration valve which caused the diaphragm not to uncover the orfice plate at high vacuum. The decel valve only opens at normal to high vacuum. With the valve malfunctioning the engine was trying to aspirate (into the 'E' port on the Rochester vari-jet) carb solely through the pcv valve ---- not enough air yielded rich burn, but with the valve not working: low vacuum .... thus the paradox.Other: Stalling on left turn does NOT involve the carb float !!!!!!!!!! Another anomaly also solved in the step by step search of the vacuum problem (and a problem that has been posted here many many times): Engine would stall when turning left (not right). The 'usual' suggestion is to increase the float height in the carb ..... WRONG WRONG WRONG as that will only cause the carb to dump gas into the intake manifold when you shut down and totally flood it and you still stall when turning left. The solution is a freakin teeny pressure switch on the high pressure side of power steering pump .... when the pump goes over pressure the switch (also) energizes the throttle kicker. $30.00 new switch and I can turn to left again!!!! You might want to remember this info for those who have 2.8L engines that stall when turning left (not right).
Anyway thanks to all who posted suggestions even that your suggestions didnt work, I thank all of you for your help...... At least the discussions here are extremely helpful in sorting things out. At least I learned that after 300K miles you should replace the *entire* emissions control system components ... its a BIG PITA if you dont. Im now back to full power and 23+ mpg for long haul runs at speed. Im now planning to make at least 500,000 miles on this XJ Cherokee beast (- currently doing a semi-frame-off restoration). Too bad DaimlerC cant build a vehicle to that standard, or without costing $100,000 across the parts counter !!!!!!!!