K rules are like this, remove all the spark plugs so it spins over easily. with the throttle wide open and a good battery(or on a charger to keep all readings the same if Batt is weak) spin it over and record the first puff. then spin it again and record what the final compression is, shouldn't take more than four compression cycles (watch the needle jump). Remove the tester drop a few drops of oil in and re spin.
Results as follows:
If the first puff is less than 80% of top compression than the motor is less than healthy,
if all are low but oil squirts bring readings up then rings are toast
If oil doesn't bring the readings up check for internal damage (valves, head gasket)
this is especially true in two adjacent cylenders but you already knew that.
More effective and with out having to spin over is a leak down test
it's a pressurizing spark plug end that will tell you where the leak is going to.
rotate the motor to Bottom Dead Center of the intake cycle, (valves closed)
And see how long it takes to lose pressure. then go and find the leak.
If it bubbles at the Radiator the head gasket is gone
If you can hear it at the valve cover it's rings or a cracked head or cylinder.
check at the tail pipe for sound and the intake plenum and you will find bad valves
Other than that when buying a JY motor smell it to see if the oil was burnt out and if there are any metal bits in the oil.
By the way try disconnecting the coil and running the starter, I doubt it will spin the motor to 1,200 rpm more like 300-600 NMc