How To Finish Motorcycle Compression Test?

First of all, we have to know the reason why should we perform a compression test?

To be frank, a compression test can determine whether the motorcycle engine can generate adequate pressure which is needed for combustion in no time. If you want to repair a motorcycle like a 12v 8ah battery motorcycle that has a loss of power, a compression test should be first on the list.

After that, you should know which compression gauge should you choose to use?

The compression gauge that you can use may be inexpensive so that even normal people can afford it. What’s more, if you want to make sure the end that screws into the cylinder head is equipped with a Schrader motorbike valve, you should be patient to check it. Of course, you may make the mistake at the beginning of buying a cheap gauge which does not have this specific merit.

Wet Test vs Dry Test

To be honest, nearly all service manuals state tests should be performed while the motorcycle engine is hot and dry. And a wet test can be performed if a dry test can not give you the right results. You can add a small amount of oil through the spark plug hole so that it can wet the cylinder wall, ring, 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 on a motorcycle like right-side shifter motorcycles?

The process to perform the compression test is a bit easy. To start with, the engine should be hot during testing. If you are working on a non-running rocker cover motorcycle, you can test while it is cold. However, values can be 10-15psi lower in some cases.

Then, you should turn petcock such as dirt bike petcock to OFF position and run motorcycle until it runs out of fuel. For fuel-injected models, you need to remove the fuel pump fuse.

Next, you have to remove all spark plugs to prevent any harm and remove the ignition fuse, if it is equipped. After that, you can hook up a compression tester to a cylinder of a motorcycle such as a valve cover gasket motorcycle. It will prolong O-ring life if you use a dab of grease or Vaseline to lube the threads and O-ring on the adapter.

You should hold the throttle all the way OPEN and crank or kickstart the straight handlebars motorcycle until the needle stops climbing usually 3-4 kicks. You can record the result carefully and compare it to value in the service manual and you can repeat steps 4-7 for all remaining cylinders.

So how can you know what do the results indicate? Generally speaking, motorcycle engines will need at least 100psi to run, and they might not run well at this low on compression. Most healthy float valve motorcycle engines will have 120+psi.

On a 4 stroke, compression could be lost through the intake or motorcycle valve, piston rings, or head gasket. On a 2 stroke, the piston ring or the head gasket.  As you can see, a compression test will simply tell you whether you have adequate compression or not. And it does not indicate where the problem lies so as to pinpoint the problem on a 4 stroke, a leak down test will have to be performed.