RichP said:
Carbon on the valves can cause the same low readings buy not allowing the valve to close all the way. How do the plugs look ??
The needle on a vacumn gauge will jump around if the valves aren´t seating. Unequal compression will make it wag some also. But like the guy said a couple of squirts of oil in the spark plug hole and higher compression readings, usually means rings.
Curious as to which cylinders, are high and low. Plugs can tell a lot, oil deposits on the plugs. A few cylinders with very low compression, if the rings are worn, will usually create excessive blow-by. If your crankcase ventilation tubes are clean, there should be very little pressure at the oil filler hole, put the palm of your hand on there and work the throttle a bit, if the blow-by pressure is there, you can feel it, if it´s serious, it will blow your hand right off of the filler hole..
There are some other fairly easy tests, that might help to narrow down the problem. Nice to have an idea what you are gonna need for parts, before you start tearing into the motor.
My motor has unequal compression, though not as low as yours, mains are loose, some piston slap, blew the doors off of a guy with a hot rod Subaru this afternoon. Jeep motors will often run for years, in a pretty loose condition. Though over reving is not recommended.
