How to Check the Compression of a Yamaha 660 Raptor

Written by chris gilliland Google
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How to Check the Compression of a Yamaha 660 Raptor
A damaged or worn piston can affect your Raptor's internal compression. (piston de scooter image by Vetea TOOMARU from

If your Yamaha Raptor 660 has lost a noticeable amount of power, it may be suffering from a loss of compression. Normally, the piston within your Raptor's motor pulls in a fresh charge of air and fuel, compressing and squeezing it into a highly-pressurised charge that is ignited by the spark plug. To operate properly, your Raptor must have a compression pressure between 163 to 210 psi. Higher than normal pressure is a sign of a damaged or worn piston. Likewise, reduced compression pressure could be caused by a damaged piston ring, valve or cylinder head gasket.

Skill level:

Things you need

  • Socket wrench
  • Spark plug socket
  • Compression gauge

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  1. 1

    Start your Raptor 660 and let it warm up to its operating temperature. This normally takes between three to five minutes if the ATV is idling in place. Stop the motor and turn off the ignition once your Raptor has warmed thoroughly.

  2. 2

    Unplug the high-tension wire from the spark plug, located on the front of the motor's cylinder. Remove the spark plug with a socket wrench and a spark plug socket.

  3. 3

    Screw the threaded end of your compression gauge's hose into the cylinder's spark plug hole. Press the release button of the side of your compression gauge to reset its indicator needle to zero.

  4. 4

    Turn your Raptor's ignition switch to the "On" position. Push the throttle lever forward to open the throttle completely. Press and hold the starter button to turn the motor over and build up compression within the cylinder. Release the starter button after two to three seconds.

  5. 5

    Take note of the cylinder's internal pressure indicated by your compression gauge. If the motor's compression is above 210 psi or below 163 psi, remove the compression gauge and squirt five to six drops of 10w40 motor oil into the cylinder. Reinstall the compression gauge and take another measurement.

  6. 6

    Unscrew the compression gauge from the motor. Reinstall the spark plug with a spark plug socket and a socket wrench. Press the high-tension wire into place over the spark plug.

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