I recently published a blog post about the importance of performing a compression test for diagnosing the overall health of a non-running motorcycle you may be inclined to bargain purchase off Craiglist/eBay.
Obviously, the logical inter-related blog post topic is how to perform a motorcycle engine compression test.
The task is not difficult and a compression tester is relatively cheap off sites like Amazon. I purchased a new one last night for less than $25 bucks. I will let you know how it is once I have had a chance to “test” it out (no pun intended). Okay, so let’s get into the details of how to perform a motorcycle engine compression test.
I want to break down the motorcycle engine compression test into 5 Simple Steps. This should keep it manageable for anyone and an easy to read post on your cell-phone/tablet while out in the garage.
Step 1: Remove the spark plugs and disable the ignition system by either (a) disconnecting the primary wires to your ignition coil, (2) removing the ignition fuse (if you have one), or grounding the spark plug wires
Step 2: Screw the hose-end of the motorcycle engine compression tester into the cylinder you are interested in (finger-tight). Good compression tester kits come with various adapters allowing for the testing of a wide variety of engines.
Step 3: Open the throttle fully (i.e twist the throttle open at the handlebar), and simultaneously……
Step 4: Turn the engine. This can be done in three ways: (a) Using the kickstarter, with the engine in neutral (2) Via the electric starter button (pressing it four or five times), and (3) Putting the engine in a high gear, with the rear wheel elevated, and turning the rear wheel. The first two methods are probably easiest to do by yourself. The last method I would reserve for a motorcycle that has no kickstart and also has wiring problems. I tend to work on old bikes with kickstarters, so method one is usually my preference.
Step 5: Check the tester gauge, record the reading, release the pressure from the tester, and repeat the process several times (you are looking for a consistent result a few times in a row). Repeat the entire process for the other cylinder.
Interpretation of Motorcycle Engine Compression Test Result: (1) The final result should be between 125 to 150 PSI per cylinder (refer to your motorcycle service manual for a more precise specification), and (2) On a multi-cylinder engine, the reading for each cylinder should be within 10 PSI of any other cylinder. You can take the interpretation further, performing various other procedures to narrow down the cause of your low compression; although this kind of work never made sense to me. If an engine has low compression and I am ripping it apart down to the crankcase, I am going to replace all vital components while I am in there (pistons, rings, valves, etc). Work smarter, not harder. Further troubleshooting or narrowing down of the culprit for low compression seems like a waste of time.
Note that doing the test on a hot engine would likely result in a few more PSI (~5% higher give or take), but doing the test on a hot engine is obviously problematic (very easy to burn yourself). You get to the same important information doing a cold test, so performing the test with the engine hot isn’t really worth the risk. Also, if the final result of the cold test is so borderline that you are “counting” on those few additional PSI, you should be rebuilding the engine anyway.
Anyways, this is my post regarding how to perform a motorcycle engine compression test.
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