How to do a Compression Test

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If your motorcycle won’t start or isn’t running well, it could be do to lack of engine compression.

A compression test is easy to perform and can help diagnose what’s going on inside your top end.

Symptoms of Low Engine Compression

There are some signs that indicate when you may want to run a compression test.

Symptoms of low compression include:

  • Misfires.
  • Backfires.
  • Fouled spark plugs.
  • Not being able to start the bike.

Before running your compression test, be sure your engine’s timing and valve clearances are set properly.

What is a compression test?

Remember, any combustion engines requires 3 main ingredients to run correctly: a fuel/air mixture, spark, and compression.

Fuel/air is handled primarily by the carbs, spark by the spark plugs and ignition system, and compression by the cylinders, pistons, valves, and the timing of the camshaft and crankshaft.

Compression describes the pressure imposed on the fuel/air mixture to allow it to ignite.

As the piston travels up the cylinder during the compression stroke, the fuel mixture is pressurized and ready for ignition.

A compression test is used to see exactly how much PSI each cylinder can produce.

Can you do a compression test by yourself?

With the right tools and know-how, you’ll be able to conduct your own compression test.

You’ll need:

  • A compression test kit.
  • Socket wrench.
  • Compression testing adapters, if necessary.

How to do a Compression Test on a Motorcycle

To run a compression test on your motorcycle engine:

  1. Remove all of the spark plugs.
  2. Turn off the ignition or ground the spark plug wires.
  3. Turn off the fuel valve or fuel pumps.
  4. Remove the spark plugs.
  5. Insert the pressure gauge hose fitting into the spark plug hole of the cylinder you want to test first.
  6. Hold the throttle open.
  7. Kick the kick starter or crank the engine over with the electric starter a few times (about 3-6 cycles or until the pressure needle stops climbing).
  8. Record the pressure reading, release the pressure in the gauge, and repeat until you get consistent results for the cylinder.
  9. 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. A percent variation of 15%-20% or more between cylinders indicates issues with the combustion chambers.

Wet Compression Test

You can further diagnose compression issues by running a wet test.

This involves first dropping about a tablespoon into the cylinder(s) with the lowest readings from the dry test.

With the oil in the cylinder, run the test again.

If the compression increases significantly, the issue has to do with a problem with the piston rings sealing against the cylinder bores.

If the readings don’t change too much, the issue can likely be traced to the valves, valve seals, or valve seats.

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 engines on a motorcycle 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.

Can you test compression by hand?

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.

While a hand compression test isn’t as accurate as using a compression test kit, it can be a quick way to rule out the issue.

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 piston rings/cylinder walls.
  • Leaking valves or bent valves.
  • Blown head gasket.
  • Improper timing.

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.

If you still get low or variable readings from the compression test, you can perform a leak down test next.

Cheap Compression Testers

If you’re using a cheap pressure gauge (i.e. Harbor Freight Compression Tester), keep in mind that 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.