A compression gauge is well worth the investment. Prices range from a bit less than £10 for a basic hobby model through to £100 or more for the professional kit. For our purposes, the budget model is generally very satisfactory. I prefer the type with a set of threaded connectors and a flexible hose. The type with a rubber end that is pressed against the spark-plug hole requires the participation of an assistant and so is less convenient.

Check your valve clearances to make sure all valves are closing completely on the compression stroke.

If you use a Colortune, remember that poor combustion symptoms do not always indicate ignition or fuel problems especially if only one or two cylinders are affected. Low compression will also lead to poor combustion. Check the compression before you start messing with the mixture or pulling the distributor to bits.

The test is best performed on an engine at working temperature, so you can do it after a run or just idle it to get the engine hot. This ensures that all parts have expanded to normal running conditions. On a cold engine, the readings may not be wholly representative of normal running but you will still get comparability between cylinders, which is, in itself, useful data.

Remove the spark plugs (carefully if they are hot! A socket that grips them internally is good for this.) Lay them out in order so that you can look at the colour of the electrodes later and maybe spot corroborative evidence of a problem cylinder. Hopefully, your plug leads are already marked but, if not, mark them for replacement in the right order.

Before you start, have a piece of paper and a pencil handy to record your results. You can prepare it by listing the cylinders 1 – 4 with spaces for the compression readings.

Your gauge will probably come with instructions, but this is how the testing is done:

1. Check that the gauge is reading zero. Press the release button to clear the gauge.
2. Fit the connector to No.1 cylinder just tight enough to compress the seal.
3. Fully open the throttle to ensure maximum airflow to the cylinder, then turn the engine over five or six times on the starter.
4. The gauge will show the maximum compression achieved. Note the figure against cylinder 1, release the pressure in the gauge to zero it, then repeat the process on cylinders 2 – 4.

When you analyse the results (see “Interpretation” below) and all appears well, then no further testing is necessary. However, if you find anomalies, then it is worth performing a second stage test. The procedure is largely as described above but before you test each cylinder, you inject about a teaspoonful of engine oil into it through the spark-plug hole. Record the second readings next to the first so that they are easily compared. This will help you diagnose the state of your piston rings. Your second, oil-assisted readings will generally be higher than the first.

Interpretation of the Results

The “normal” optimal pressure range for our type of engine is about 150 -160 psi, so if all your readings fall within or close to these parameters (allowing for possible inaccuracies in cheaper gauges), your engine is in good nick at the top end.

1. The more similar the readings are on each cylinder, the better. This indicates that your engine will run with good balance.
2. If your test reveals a range of pressures, then do not be too concerned as long as there is no more than 20psi between the highest and the lowest. Alternatively, calculate the average and check that no cylinder is more than 10psi adrift of this. The higher the state of tune, the more important is this comparison.
3. If your engine is newly rebuilt, you may get readings outside the normal range while the components are bedding in. Check again after it has been run in and has done a few miles.
4. An engine that delivers higher readings may have been modified with flat pistons or its cylinder head may have had a significant amount of metal skimmed off the gasket face. Bear in mind that this suggests a higher compression ratio and that you may benefit from higher octane fuel.
5. A particularly low reading on one or more cylinders should prompt you to go on and perform the oil-assisted test. If the oil significantly increases the pressure on the second reading, then the problem is worn piston rings or bores. If there is little difference, then the problem is more serious: blown gasket, burnt valve, holed piston or broken rings. These problems will normally also cause poor performance and/or rough running. A cylinder normally needs 100psi or more to fire properly, so with pressure much lower than this, that cylinder is hardly firing and is contributing little to the power output.
6. If you find that two adjacent cylinders are reading low, it may indicate that the head gasket is leaking between them. This is common on the two middle cylinders of B-series engines, where the land between the cylinders is narrow and vulnerable. With this symptom, the oil-assisted test will not improve the readings.
7. If low readings from worn bores or rings are produced on a pair of cylinders that are fed by the same carburettor on a multi-carburettor set-up, then it suggests that this carburettor may have been running too rich and the excess fuel has been washing the oil from the bores.

Examples

158-152-155-150                   All is well

158-152-138-134 (Test1) 160-158-150-149 (Test 2+oil) Worn bore or pistons on 3 & 4

158-152-85-155 (Test1) 160-154-87-158 (Test2) Valve, gasket or piston problem on 3

158-82-85-150 (Test1) 160-83-87-154 (Test2) Probable gasket leak between 2 & 3

Think of compression testing as taking your engine’s blood pressure. Do it routinely and often and it will either reassure you that all is well or alert you to early symptoms of a problem. If you are taking the plugs out anyway to check their condition, it scarcely adds any time to routine maintenance and the kit is not expensive. Keep a note of the results so that you can spot changes and trends beginning to happen.