what is the compression suposed to be on a 1983 jeep cj7 258 straight 6 cylender

You should get around 139 psi. More importantly is that they are all very close in pressure. If they are all within 10% of each other that is best. If you really want to get a good indication of the condition of your engine and its components the vacuum check is the best. Get yourself a vacuum gauge and hook it up to manifold vacuum........ Here is a good reference for vacuum trouble shooting.......... You should get between 17-22 in. hg. depending on your elevation above 2000 feet MSL or mean sea level. To perform this test hook vacuum gauge directly to manifold vacuum. 1. Low steady needle usually indicates leaky intake, a leaky vacuum hose, late timing, or incorrect camshaft timing. 2. If reading is 3-8 inches low and fluctuates at that low reading suspect an intake manifold gasket leak at intake port or faulty injector. 3. If the needle has regular drops of about 2-4 in hg. at a steady rate the valves are probably leaking. (Perform a compression check). 4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire. 5. A rapid vibration of about four in hg. indicates worn valve guides. 6. Slight fluctuation of one inch or so indicates ignition problems. 7. Large fluctuation perform a compression check. 8. If needle moves slowly through a wide range check for PCV issues.