Cylinder Balance Test
A cylinder power balance test is performed to locate the cylinder causing a lack of power and not contributing to the engine’s balance and performance. It's often used along with a cylinder compression and cylinder leakage test to pinpoint the problem.
The test gives us an indication of each cylinder’s power, testing its effectiveness compared to the others. Each cylinder's ignition is disabled at a particular speed (800-1000 rpm), and the rpm drop is recorded.
On many engines, when an analyzer is unavailable, the technician can disconnect the plug boot from its corresponding spark plug or distributor connection long enough to disable the desired cylinder.
Sometimes an O2 sensor must be disabled to prevent the computer from compensating for the sudden drop in rpm. If unsure, check with manufacturer's specifications before beginning. Sometimes it's best to use a grabber, or a tool that will protect your hand from any shock that may result from grasping and removing the plug cover.
Notice the drop in rpm as the boot is removed from the spark plug tip. If a cylinder's rpm drop is not consistent with the others, this indicates a problem with that cylinder. An engine analyzer does the same thing as this manual cylinder power balance test. It tests each cylinder and compares the results.
The reason's for a cylinder not contributing include an ignition problem, fuel delivery, or a mechanical problem such as a faulty valve or worn piston rings. As this question suggests, there may be more than one cylinder involved. The question left out a faulty head gasket, cracked head, or block as a possible cause for low results on adjacent cylinders.