2. Carrying out a compression test
- A good idea of the internal state of the engine can be gained by testing
its compression as follows.
- The engine must be fully warmed up to normal operating temperature and the battery fully charged for the test results to
- Remove all the spark plugs. Noting the warnings concerning servicing the ignition system given in
Chapter 6 - Ignition, lay the spark plugs
on the cylinder head so that their metal bodies are securely earthed to the metal
of the cylinder head (to prevent damage to the ignition system) and so that their electrodes are
well clear of the spark plug orifices (to prevent the risk of sparks igniting any fuel/air mixture that is
ejected). While one cylinder is being tested, place a wad of rag over each of
the remaining spark plug apertures as additional protection.
- Attach an accurate, good quality compression gauge (tester) to the cylinder head spark plug orifice, following its manufacturer's
instructions. Open the throttle fully. Spin the engine over on the starter
motor and note the readings recorded.
- After one or two revolutions the pressure should build up to a maximum
figure and then stabilize; note the reading and repeat the test on the remaining
cylinders. There should be no discernible difference between any readings. The expected pressures are given in
Specifications. If all pressures are the same and in the good or normal range then
the engine is in good condition.
- lf there is a marked discrepancy between the readings, or if any is in the
poor range, the appropriate cylinder must be cheeked carefully.
- Note that during a normal compression test one would go on to temporarily
seal the piston rings by pouring a quantity of oil into the barrel and then take a second set of readings. lf the pressure
increased noticeably it could then be assumed that the piston rings were worn
rather than the valves. Since it would be very difficult to get a full
seal from such a method in a warm flat-cylinder engine there is little
point in doing this; check the pistons and rings as well as the head gasket and
valves when looking for the cause of compression loss.