What is a Compression Test?

A compression test measures the peak pressure in the engine’s cylinder as the engine is turning over or a reduced cylinder pressure value if the engine has a decompression system.

Compression Testing Methods:

There are two different ways that you can test the engine, warm or cold. “They say” the test should be done after the engine is warmed up. This allows for the bike to reach operating temperature and give a more accurate compression test. The piston, cylinder and rings will expand with heat. This can cause these parts to represent what functionality they would have while you are riding.

This is a great idea in theory but becomes difficult once you have spent the time it takes to remove the seat and gas tank to access the spark plug. You will need enough space for the spark plug to come out and the compression gauge to connect.

At the end of the day the most important thing when doing compression tests is to have repeatable results. If you feel like you can get the engine warm and complete the test fast enough then go for it. Many mechanics swear by this method. However, many people perform the tests cold as well and have good results. Go with what you feel is more accurate and consistent for you. 

Decompression System:

Some engines have a decompression system of some kind, which adds a whole new variable to the test. In order to do a test on an engine with a decompression system, you must remove or disable it. If you are working on a bike with a decompression system that bleeds out cylinder pressure as the engine is kicked over, you may want to perform a leak down test instead. This will give you much more accurate information about the engines health

Best Compression Testing Techniques:

Use the same brand testing gauge every time you perform the test. Many brands will display different results and can really make your head spin trying to figure out which one to go with. This is really just a different calibration of the gauges. Use the same gauge and get the same results for every test you do.

A 50cc, 60cc, 80cc, 100cc, 125cc, 200cc or 500 cc should be able to output roughly 120-240 PSI. A 250cc should be able to reach around 170-240 PSI. Consult your owners manual to see if they list the compression value of your motorcycle, this is usual a range of PSI.

Do a compression test on your bike when you get it, whether its new or used and save the PSI number somewhere. Use this number as a reference in the future when you do another test to determine whether or not you are losing or gaining compression. I like to get a note book for each bike in the garage with all sorts of details about maintenance and stats such as compression. 

Higher altitudes will report a lower compression reading, so don’t freak out if you get different results at different altitudes. This is due to a lower air density at higher altitudes, therefore less air to compress.

Prep Before Compression Test:

• Make sure the motorcycle is very clean, it’s very important not to allow dirt or water anywhere near the spark plug or coil pack on 4 strokes. If dirt does get in the cylinder it can score the cylinder walls and lead to faster wear on the piston and rings. Use compressed air to blow around the spark plug prior to removal of the cap and plug. Just do your best to make sure it’s clean.
• Turn the petcock valve into the off position (if you have one, see image).
• Check the air filter to make sure it is clean and in good condition.
• Most Bikes will need you to remove the gas tank for a direct access to the spark plug. This will make sure you don’t kink the hose and cause inaccurate readings on the compression gauge. Use a rag to catch the fuel from the fuel line connected to the gas tank.
• Now you just need to remove the spark plug and attach the compression tester. Here is the one I use from Amazon. There will be different adapters for the testers so figure out which one matches your bikes threads. Thread the tester on until finger tight and the 0-ring or gasket is compressed. Make sure this is snug and sealed well or you could have a compression leak, leading to inaccurate readings.

How to Perform the Compression Test:

We are now assuming you have done all of the previous steps and are ready to begin testing the compression. You will need to have the engine run switch turned to OFF (if you have one) or have the kill button pressed the entire time. If you are working on an electric start bike you may need to leave the engine run switch turned ON so that the engine can turn over.

Hold the throttle wide open and kick the bike over 5 times as fast and hard as you can. Check the compression tester for the value recorded. 

Reset the compression tester gauge and repeat the test 3-5 more times to check for consistent results. Write down the PSI of each test in a maintenance log so that you can look back at them the next time you perform a compression test.

This concludes how to do a compression test. They are very easy to perform so don’t be afraid to try it yourself! Thanks for reading, leave a comment and let me know if this helped you or if there is anything you would add!

Hey! Go check out my article on how to winterize a motorcycle!

-DirtBikeMasters.com