There are several situations that may prompt you to test the health of an engine:
You’re buying a used motorcycle and need to know if it’s worth investing in
Your bike is misfiring
You’ve noticed a lot of smoke from your exhaust pipe
Your bike has gradually been losing power
You want to ensure your bike’s internal systems are running well
A compression test is a great way to gauge how your motorcycle is doing without dismantling the whole thing.
So how does a compression test work? It measures the pressure of your bike’s cylinders. Proper pressure is essential to making sure your engine’s moving parts are working correctly. Low pressure readings can indicate the presence of damage that should be addressed.
While you can have a mechanic perform the test, those of us who prefer to work on our bikes can easily do it at home. It’s a fairly simple process that requires only a few pieces of equipment:
A compression tester
Your bike’s manual
Protective gear such as hardy gloves and eyewear
Remember that just because this process is easy doesn’t mean that you shouldn’t be careful. There are several steps you need to follow to ensure that compression testing is done safely and accurately. To help you out, here are the basics of how to do a compression test.
1. Get a Compression Tester
If you don’t already have a motorcycle compression tester, you’ll need to purchase or borrow one. The good news is that many compression testers have multiple attachments to ensure they can be used with most bikes.
Buying a compression tester can be a great investment, since tester kits are relatively inexpensive and can be used on multiple kit you choose, doing your own compression tests can end up saving you money as well as a trip to the shop.
2. Prepare Bike for Testing
There’s some debate on what condition your bike should be in during the test. The decision really boils down to what you prefer and what you’re trying to determine.
Cold vs. Hot
Should you run your bike before performing a pressure test? Some mechanics advocate to test cold, since this eliminates the issue of trying to work on an engine that is physically hot and may cause burns.
However, testing cold usually results in lower readings. Testing a hot bike gives you better accuracy, since the internal conditions are closer to what they would be on the road.
Dry vs. Wet
Testing your bike dry means you don’t add any additional oil to the engine before starting the test. This is the method you should initially use to identify if a pressure problem exists. If you get low readings, a wet test can be performed to see if worn piston rings are the cause.
To test wet, add a little oil to the cylinder. This should temporarily form a seal between the piston rings and cylinder wall. Perform the compression test again – if the results are higher, your piston rings are likely the problem and need replacing. If you still get a low reading, there’s something else causing the issue.
3. Disable Fuel System
Before you actually attempt the test, it’s essential – and we cannot stress that enough – to disable your ignition and fuel systems. Failure to do so can result in fire.
To disable these systems, consult your manual to determine the appropriate fuse or relay. Once the test is complete, it should be a simple process to reconnect the systems.
4. Connect Compression Tester
Your next step is to connect the compression tester. The attachments usually fit into a spark plug port. To ensure the most accurate readings, you’ll need to remove all of the spark plugs. This is also a good time to ensure that your battery is fully charged. For best results, your battery should be fully charged during every test. This may require you to charge it between cylinders.
If there’s a rubber seal on the gauge fitting you’re using, make sure there’s no damage to it, or you may receive inaccurate readings. When you’re ready to connect the compression tester, simply screw the gauge fitting into the port.
5. Open the Throttle
Before you crank the engine, you need to open the throttle all the way. This allows air into the engine so the pistons can move. Failing to open the throttle can yield false low ratings.
6. Turn the Engine
Kickstart your engine until the reading stabilizes. You may have to crank the engine several times to achieve this.
Keep track of how many times you have to kickstart to get a reading on the first cylinder. You should crank the engine that many times for each subsequent cylinder to make sure all readings can be accurately compared to each other.
7. Record Readings
This may seem like a no-brainer, but we’ve included this step as a reminder that it’s better safe than sorry. While some of you out there may have perfect memories, we sure don’t. To prevent you from having to do a test twice, we recommend writing down the PSI. If you’ve got an analog valve, you can also mark each reading directly on the equipment with a piece of chalk.
8. Determine Your Results
Readings will vary by make and model, so for an exact ideal measurement, we recommend consulting your manual. However, if you’re just looking for a ballpark, pressure should be from 125 PSI to 150 PSI in each cylinder, and each cylinder should not vary by more than 10 percent from the others.
If your results fall within this range, then great – your engine is in wonderful health. If you have low results, something likely needs to get replaced. In the worst case scenario, the whole engine might need to be rebuilt. For those not familiar with dismantling an engine, we recommend having a mechanic take a look.
Knowing what’s going on inside your cylinder is a great way to look after your bike’s overall health. With a small investment in a compression tester, you can look after your motorcycle for years to come.