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.
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.
5. Hold the throttle wide open
7. Check the value recorded by the compression tester.
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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