CX / GL 500 / 650 Compression Test

This page shows you how to do an engine compression test, which measures the amount of pressure inside the combustion chamber. This test is an indication of wear on the pistons, rings, bores and valves.

Skill : 1. Personally dirty : 1. Work mess : 0. Space : 1. Time : 15 minutes.

Skill levels explained.

All nut and bolt sizes are given for the spanner size required to fit them.

Tools : spark plug wrench and tommy bar; compression test meter.

I am very receptive to comments and suggestions, but you use these pages entirely at your own risk.

The compression test meter I used (left) was calibrated in pounds per square inch (psi). I'll give the recommended limits in both psi and kg/cm².

At the "engine" end it has a tapered hard rubber cone with a hole through the central axis (right).







At the other end is the dial (left). Make sure this is zeroed before you do each test. Some meters have a press-button which flicked the dial back to zero; this one just gradually subsides back to rest after you use it.









This test must be carried out with the tappets correctly adjusted and the engine at normal operating temperature; either take the bike for a 10 minute run or allow it to idle for 20 minutes, until the temperature gauge is vertical. The test is carried out on one cylinder at a time.

Stop the engine and remove the sparking plug (right); careful, it will be hot (as if you didn't know that).






Replace the plug in the plug cap, and earth the plug on some convenient metal place (left). This prevents damage to the ignition system. DON'T USE THE ROCKER BOX COVERS AS THESE ARE NOT PROPERLY EARTHED.

Move the engine kill switch to "off" (right).







Turn on the ignition and insert the engine end of the meter squarely and firmly into the spark plug hole. With the twistgrip at full throttle (right), spin the engine for 5 seconds. You may have to push down hard on the meter to keep it in the spark plug hole.




Read the pressure from the dial gauge of the meter and note the reading. Now zero the meter and repeat the test twice more, taking the average of the three readings. These should not vary much.

Photo (left) shows the gauge starting to read up the scale as the engine is turned over onthe starter motor.

Replace the spark plug and its cap, and repeat the test on the other cylinder.







all engines 500cc and 650cc
171 psi +/- 28 psi (12 kg/cm² +/- 2 kg/cm²)
143 psi (10 kg/cm²)
199 psi (14 kg/cm²)

Readings below this suggest:-

1. Valves leaking (decoke and regrind).

2. Excessive piston ring wear.

3. Excessive piston/bore wear.

4. Blowing head gasket, although this usually manifests itself in more obvious ways, such as water / oil contamination, exhaust stains and leaks around the cylinder head, power loss.

To determine whether the compression loss is due to valve or piston/ring/bore wear, pour a teaspoonful of engine oil into each combustion chamber and repeat the test. If the compression rises, the wear is either rings, pistons, or bores; this requires major engine surgery as you can't get to the pistons or rings without almost completely stripping the engine.

If the compression stays as it was, it's valve wear, requiring a decoke. This is not a difficult job

By way of a temporary fix, buy some Redex or other upper cylinder lubricant and follow the manufacturers' instructions on how this can help the problem. It isn't a full fix, but it does help remove contamination from the combustion chamber.

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