Compression Testing

Here we demonstrate how to test the compression of your lawn mower

Tools/Parts Required 

  • Spark Plug Spanner
  • Compression Tester

Step 1.

Firstly we remove the spark plug; then screw in the compression tester.

Step 2

Next pull the starter cord 4-5 times to get an accurate reading. Anything above 80 psi is great, anything less and it’s likely your piston and rings need replacing.


  1. Hi, I have done a test on my mower and have found that with the decom valve still in place the compression only goes up to 50-60 psi, but with the valve replaced with a plug it reads over 90psi. I have tested the valve and it seems to be working fine. The mower is also working fine. Any ideas on why the readings are so different?
    Thanks Ivan

    • Hi Ivan

      I would think that your decompressor is a little worn/stiff so when you are using the pull start to test compression it is not properly closing. Once the engine starts the pressure would be much greater and it is likely the decompressor then closes properly

  2. Over changed the oil, spark plug & gauged correctly, replaced the primer bulb, cleaned the carburetor, & replaced all fuel lines but my mower still won’t start. The bulb keeps sucking in air only..HELP any idea why?

    • The mower needs three things to start, A good spark, Fuel in the cylinder and good compression. If you check for these three things you can usually determine which is causing the problem and from there figure out what may be causing the issue.

    • Put a piece of tape over the primer bulb, often the diaphragm to that primer bulb goes bad and sucks more air in than it’s supposed to, garuntee it’ll start afterwards

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