A compression test can quickly determine if the engine can generate adequate pressure needed for combustion . If you are trying to repair a motorcycle that has loss of power, or simply won’t start, a compression test should be first on the list.
The gauge that I use is inexpensive and posted information about it here. Be aware, you want to make sure the end that screws into the cylinder head is equipped with a Schrader valve. I made the mistake of buying a cheap gauge, and it did not have this feature. As a result, readings were much lower than actual since the long hose supplied added to the cylinder head volume.
Nearly all service manuals state tests should be performed while engine is hot and dry (meaning no addition of oil). A wet test can be performed if a dry test yields low results. Adding a small amount of oil through the spark plug hole will wet the cylinder wall & rings and provide a temporary seal. If the wet test results in a higher reading, the piston rings are worn.
The process to perform this test is really easy. The engine should be HOT during testing. If you are working on a non-running motorcycle, you can test while cold, however values can be 10-15psi lower in some cases.
So what do the results indicate? Engines will need at least 100psi to run, and they might not run well at this low on compression. Most healthy motorcycle engines will have 120+psi. I would investigate further with any readings under 110psi. Readings on all cylinders should be within 10% of each other.
What if results show low compression or cylinders are not within 10% of each other? Well, it will need further investigating as to what the problem may be. On a 4 stroke, compression could be lost through the intake or exhaust valves, piston rings, or head gasket. On a 2 stroke, the piston ring(s) or the head gasket. As you can see, a compression test will simply tell you whether you have adequate compression or not, it does not indicate where the problem lies. To pinpoint the problem on a 4 stroke, a leak down test will have to be performed.