It means the engine is getting tired. Buy, rent or barrow a compression tester and see what you've got. Be sure to pull all the spark plugs, throttle held wide open, crank the engine and record the 5th reading on the tester. Do this for every cylinder. Then add a tablespoon of oil to the cylinder and do it again. Fairly simple to run. Write everything down and let us know the results.
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1979 CJ7, FI 5.0L Ford, NP435, D300, Full floated D44 Detroit, D30 Detroit EZ Locker.