If your motorcycle won’t start or isn’t running well, it could be do to lack of compression. A compression test is easy to perform and can help diagnose what’s going on inside your top end.
Reading Motorcycle Compression Test Results
Generally speaking, a motorcycle with around 100 PSI compression in its cylinders will likely need a rebuild very soon.
Anything less than 100 PSI is going to have issues running, if it can even start at all.
Most healthy motorcycle cylinders should read at least 110-120 PSI. All readings should be about the same.
Compression gauges work by either screwing into or pressing into your spark plug holes with a hose attached to a pressure gauge. The side of the gauge usually has a pressure release button.
If you don’t have a compression gauge, you can run a poor-man’s compression test by holding your thumb on top of the spark plug hole.
If you’re able to keep your thumb on the hole while you turn the engine over, your engine has low compression.
How to do a Compression Test on a Motorcycle
To run a compression test on your motorcycle engine:
- Remove all of the spark plugs.
- Turn off the ignition or ground the spark plugs.
- Insert the pressure gauge hose fitting into the spark plug hole of the cylinder you want to test first.
- Hold the throttle open.
- Kick the kick starter or turn the engine over with the electric starter a few times (about 3-4 cycles or until the pressure needle stops climbing).
- Record the pressure reading, release the pressure in the gauge, and repeat until you get consistent results for the cylinder.
- Move on to the next cylinder.
A warmed up engine will give you a more accurate reading, but you can run a compression test on a cold engine.
A good test will result in decent compression/PSI. Your shop manual should specify the exact readings for your bike. Additionally, you’ll want all of the cylinders to be within about 10 PSI of one another.
Low Compression Test Results
If you are getting low readings on one or more cylinders, you’ll need to determine the cause before you start tearing into your engine. Low compression readings could be due to:
- Worn rings/cylinder walls.
- Leaking valves.
To check if your piston rings are causing the low compression:
- Drop some engine oil down the spark plug hole.
- Attach your compression tester gauge and run the test again.
- If the readings come back higher than the original compression test, the low compression is likely due to the rings.
- If the readings don’t change, it’s likely due to your motorcycle’s valves.
If your valve clearance is too tight and not set properly, it could be causing the low readings. Check and set your valve clearances as per the motorcycle shop manual.
Run the compression test again.
Cheap Compression Testers
If you’re using a cheap pressure gauge (i.e. Harbor Freight Compression Tester), you are probably going to get much lower readings that the actual pressure in each cylinder.
The cheap gauges typically come with much longer hoses which add to the total cylinder volume.
Motorcycle Leak Down Test
If you get low readings or variable readings from the compression test you just performed, you can performed a leak down test to further diagnose the bike’s potential compression issues.
Leak-down tests can also be used for motorcycles that feature decompression mechanisms (2-strokes).
A leak down test involves pressurizing the cylinders while the engine is a top dead center (TDC). In this position all the valves will be closed, and the only places where pressure can escape is through:
- The valve seats.
- The piston rings.
- The head gasket.
Leak down tests should be performed cold.
To perform the leak down test:
- Bring the cylinder you’re testing to TDC position.
- Remove the spark plug.
- Hook up the air pressure supply to the spark plug hole.
- Regulate the air pressure to 100 PSI.
- Listen and observe for any leaks.
- Record the pressure.
Leakage can be measured as a percentage: supplied air pressure – recorded air pressure, divided by supplied air pressure.
Leakage percentage above 15% indicates that components are likely worn past serviceable limits.
If you’ve recently rebuilt the engine, make sure you’ve torqued down the cylinder and head to the proper values in the proper sequence.