A number of things could be wrong, carb problems, eng timing, vac leaks etc. I would start with running a good quality cleaner (something like seafoam) in the fuel tank and spray some carb cleaner through the carburetor too. Also can run a can of seafoam down the carb or through a vac line too. Look up a few threads on here about seafoam, most of them are probably on the 4.0 but the application will be about the same.
The cleaning stuff mentioned above can be done easily and cheap. It could be that the carb is needing a rebuild, or simply worn out. Engine may need a good tune up too (plugs, wires, cap, rotor, fuel/air filters etc) Spray carb cleaner around vacuum lines and gaskets at carb and intake manifold to find possible vac leaks.
Two things to google are "nutter bypass" and "4.2 HEI upgrade" Those two things alone really woke up my old CJ which was basically the same engine and carb that you have.