Last week I talked about what a compression test is, touched on some of the finer details you should know about when performing a compression test, and provided information on when it makes the most sense to perform a compression test. This week I’m going to wrap up compression testing by providing a step-by-step write up on how to perform a compression test.
Before getting into the step by step how-to a reader left a comment asking for further information on compression testing bikes with electric starters. I actually believe electric start bikes are easier to test than kick start bikes because the rate at which the engine is turning over is more consistent. My recommendation would be to engage the starter for a set period of time. Around 5 - 10 seconds is usually enough time to crank the engine over sufficiently and obtain an accurate reading. When performing the test you will notice the readings on the pressure gauge will be much more consistent as the engine is turning over compared to what you see on kick start engines. The battery should also be fully charged and in good condition. Apart from using a good battery and establishing a time period where the starter is engaged there isn't much more to it.
For clarity, the above tips are only for single cylinder engines. For multi-cylinder engines additional considerations and precautions need to be taken to ensure the engine doesn't start while performing the compression test. All plug leads will need to be disconnected. If the engine is fuel injected disconnecting the fuel pump or injectors is a good idea too so that fuel doesn't build up in the cylinders or inlet tracts after repeating the compression test for each of the cylinders. For multi-cylinder engines the compression readings should be compared across cylinders. As a rule of thumb if there is a 20% difference across cylinders further diagnostic work should be carried out to determine what the problem may be.
For clarity, the above tips are only for single cylinder engines. For multi-cylinder engines additional considerations and precautions need to be taken to ensure the engine doesn't start while performing the compression test. All plug leads will need to be disconnected. If the engine is fuel injected disconnecting the fuel pump or injectors is a good idea too so that fuel doesn't build up in the cylinders or inlet tracts after repeating the compression test for each of the cylinders. For multi-cylinder engines the compression readings should be compared across cylinders. As a rule of thumb if there is a 20% difference across cylinders further diagnostic work should be carried out to determine what the problem may be.
How to Perform a Compression Test
First, I would recommend confirming if a compression value is provided by the manufacturer in the service or owner's manuals. If a value is provided, further details should also be provided on how the value was obtained. For example, some manufacturers may specify a certain number of kicks or a kicking RPM used to obtain results. Once you know this information you can attempt to recreate their test conditions as best as you can. This usually means kicking as hard and as fast as you can for a certain number of kicks! For engines featuring decompression systems which are adjusted manually, making sure the decompression system is set up and functioning correctly is very important. An incorrectly adjusted decompression mechanism can have a big effect on the compression reading. If the engine is equipped with a decompression system, but there is no information provided by the manufacturer it would be my recommendation to perform a leak down test instead. For all other engines which don't incorporate a decompression system or have an automatic non-adjustable system, proceed as follows.
Steps for Performing a Compression Test (Cold)
1. Remove the seat from the bike.
First, I would recommend confirming if a compression value is provided by the manufacturer in the service or owner's manuals. If a value is provided, further details should also be provided on how the value was obtained. For example, some manufacturers may specify a certain number of kicks or a kicking RPM used to obtain results. Once you know this information you can attempt to recreate their test conditions as best as you can. This usually means kicking as hard and as fast as you can for a certain number of kicks! For engines featuring decompression systems which are adjusted manually, making sure the decompression system is set up and functioning correctly is very important. An incorrectly adjusted decompression mechanism can have a big effect on the compression reading. If the engine is equipped with a decompression system, but there is no information provided by the manufacturer it would be my recommendation to perform a leak down test instead. For all other engines which don't incorporate a decompression system or have an automatic non-adjustable system, proceed as follows.
Steps for Performing a Compression Test (Cold)
1. Remove the seat from the bike.
2. Remove the fuel tank and radiator shrouds from the bike. Make sure the petcock is turned off, if the bike is equipped with one before removing the fuel line. Use a rag to catch any fuel draining from the line.
3. Remove the spark plug cap. Prior to removing the plug blow compressed air into the plug cavity to rid it of dust and debris so that it can’t get into the engine. Remove the spark plug.
4. Install the compression tester in the spark plug hole. Make sure all the fittings are tight so there is no chance of getting false readings.
5. Hold the throttle wide open
5. Hold the throttle wide open
6. Kick the bike over 5 times as hard and as fast as you can.
7. Check the value recorded by the compression tester.
7. Check the value recorded by the compression tester.
8. Reset the gauge and repeat the test at least three times to make sure your results are accurate and repeatable. Write your results down in your maintenance log or somewhere safe so you can refer back to them at a later date.
That sums up how to perform a compression test. Pretty straightforward and simple! If you have any tips, questions, or comments please voice your opinion, I always enjoy hearing from everyone.
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That sums up how to perform a compression test. Pretty straightforward and simple! If you have any tips, questions, or comments please voice your opinion, I always enjoy hearing from everyone.
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