Why perform a compression test?
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.
Which compression gauge should you use?
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.
Wet vs dry test
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.
How do you perform a compression test?
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.
- Turn petcock to OFF position and run motorcycle until it runs out of fuel. For fuel injected models, remove the fuel pump fuse.
- Remove all spark plugs.
- Remove ignition fuse, if equipped.
- Hook up compression tester to a cylinder. Use a dab of grease or Vaseline to lube the threads and O-ring on adapter. This will prolong O-ring life.
- Hold the throttle all the way OPEN.
- Crank or kickstart the motorcycle until the needle stops climbing. Usually 3-4 kicks.
- Record the result and compare to value in service manual.
- Repeat steps 4-7 for all remaining cylinders.
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.