COMPRESSION TEST Valuable time can be wasted trying to tune an engine which is badly worn. This is particularly applicable in the case of an engine which has covered considerable mileage. It is therefore always worthwhile checking the cylinder compression pressures first to determine the general state of the unit.
A compression tester will be required for this operation, and one of these can be purchased quite cheaply from most motor accessory shops, or even hired.
The specified compression pressures for the various models are given in 'Tune-Up Data' at the end of this section, but it should be noted that the engine must be at normal operating temperature to get reliable readings.
1. First run the engine up to normal operating temperature.
2. Remove all the spark plugs. When disconnecting the leads grasp the moulded cap and pull it off the plug. Do not pull on the lead itself otherwise the core inside the lead may be damaged or broken.
3. Push or screw the connector of the compression tester into the No. I plug hole and, with the throttle held in the wide open position, crank the engine over with the starter. If the compression tester has to be held in position by hand hold it firmly ensuring that there is no leakage of compression.
4. As the engine turns, the gauge reading will increase in steps until the maximum pressure is reached.
Note this reading carefully. The number of compression strokes, indicated by the 'pulses' on the gauge, required to reach the maximum pressure should also be noted.
5. Repeat this procedure for the other cylinders, noting in each case the reading obtained and the number of 'pulses'.
6. Compare the readings with the specified figure. If all the readings are high and within about 10% of each other this indicates that the engine is in good order, provided that the readings are close to the specified figure.
7. If one or more readings are low, the test should be repeated after injecting a small quantity of engine oil into the cylinder through the plug hole to form a seal between the piston and the cylinder wall. If a marked increase in pressure is then obtained, this shows that the leakage is mainly via the piston rings.
8. If no increase in compression is obtained, the fault must be due to leakage past the valves or gaskets.
However, it should be noted that if a cylinder has a badly scored bore, or damaged piston, the oil will fail to seal the leak and no improvement in compression will be obtained.
9. In any case, low readings will necessitate further investigation to determine the cause, and remedial action taken to correct it.