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  1. #1

    Understanding Compression Test Correctly?

    Doing my compression test this afternoon.

    This is how I'm understanding it:

    1.) Remove spark plugs but keep the plugs attached to them
    2.) Ground spark plugs on cylinder etc
    3.) Insert screw in type gauge
    4.) Press the push to start button and hold throttle wide open and keep doing this until the compression tester doesn't raise anymore
    5.) Make sure each cylinder holds pressure for a while (10-15 secs?)
    6.) Pull off quick connection unscrew compression tester and move to next cylinder
    7.) Do two rounds of this for each cylinder and average numbers?
    **If I want I can do three rounds**

    Thanks Guys.


  2. #2
    LaveyT's Avatar
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    5.) Make sure each cylinder holds pressure for a while (10-15 secs?)
    Most compression testers have a check valve to hold the highest pressure attained.
    The "Leak down" test is a bit more complex. Other than that make sure the battery is strong and you list looks pretty good!

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    The leak down test aspect is typically not necessary. The recorded peak pressure achieved by each cylinder is the data you need.

    Step 1A) Make sure the battery is not only well charged but actually internally healthy. Cranking RPM will affect the compression numbers on the gauge. A weak battery will deliver lower voltage while cranking and therefore crank more slowly and produce somewhat lower numbers.

    Related note: Do not put any 2-stroke oil into the cylinders before testing compression, unless you have a specific reason to do so. Oil will improve the piston ring sealing on the not-running engine, especially with worn piston rings or scored cylinder walls. So the added oil will produce higher measured compression, but of course the actual compression when the engine is running will not have that excess oil.

    The whole reason for doing a compression test is to look for clues regarding internal engine condition. There are several things to look for when preparing for a compression test.

    As you remove each spark plug, examine the plug. If the tip area has a gray powdery appearance, this is a sign that the aluminum piston dome is being eroded and spraying fine particles of metal onto the spark plug. This of course is not a good thing, usually indicating that cylinder is running excessively lean.

    Verify that the correct spark plug brand and type are being used. Most Polaris engines use NGK spark plugs, resistor type. Some engines use the P variant with projected tip, others use the regular plug without the P code.

    Look for signs of physical damage to the plug. A bent closed ground tang gap often means a connecting rod bearing has failed, allowing the piston to hit the spark plug tip. Rust or corrosion on the firing end is not good either.

    Look at the threads in the spark plug hole. If badly worn or damaged, the head may need to be replaced or repaired. The unseated spark plugs should screw in and out with finger power or just mild tool effort. Very tight turning effort means something is wrong.

    Use a tiny flashlight light or borescope to inspect the top of each piston by looking through the spark plug hole. Rotate the engine driveshaft coupler by hand so the piston is near the bottom of the cylinder, so you can see the entire piston top. On a properly operating engine that has had some run time there should be a distinct piston wash pattern.

    If working outside, a large towel or blanket to cover your head and the engine can block the sunlight and make it easier to see inside the cylinder.

  4. #4
    Thank you guys. I was going to go ahead and update the plugs anyways (even if they only semi used). I was looking at the NGK Laser Platinum OEM style, these look to be the best.

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by dmueller9834 View Post
    ... I was looking at the NGK Laser Platinum OEM style, these look to be the best.
    Use the exact NGK spark plug specified for your engine.

    Using a different spark plug may or may not make any improvement but it becomes another variable which can cause doubt during diagnostics.

  6. #6
    Quote Originally Posted by K447 View Post
    Quote Originally Posted by dmueller9834 View Post
    ... I was looking at the NGK Laser Platinum OEM style, these look to be the best.
    Use the exact NGK spark plug specified for your engine.

    Using a different spark plug may or may not make any improvement but it becomes another variable which can cause doubt during diagnostics.
    I went on NGK, searched by make and model and it came back with the standard NGK plug. Then it said for improving the plugs make sure you select the ones that are OEM approved.

    http://www.ngksparkplugs.com/product...x.asp?mode=nml

    ^^Bottom left standard plug.

    The plug I was looking at was the Laser Platinum plug which is directly above the standard application. I just figured they would be interchangeable because they are both OEM approved. How I'll probably just get the standard.

  7. #7
    BlueFishCrisis's Avatar
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    I'd just get the standard. What ski?

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    BlueFishCrisis's Avatar
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    Nevermind - you need NGK BPR8ES for your 1996 Polaris 750 SL
    Last edited by K447; 07-09-2014 at 12:58 PM. Reason: 1996 Polaris 750 SL

  9. #9
    Quote Originally Posted by BlueFishCrisis View Post
    Nevermind - you need NGK BPR8ES for your 1996 Polaris 750 SL
    Actually have a 780 SLX however both seems to use same plug. I got all my fluids ready to go as well however they are no fill lines, best approach I've read is to check inside the hull for each tank and fill them up?

  10. #10
    BlueFishCrisis's Avatar
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    Quote Originally Posted by dmueller9834 View Post
    Quote Originally Posted by BlueFishCrisis View Post
    Nevermind - you need NGK BPR8ES for your 1996 Polaris 750 SL
    Actually have a 780 SLX however both seems to use same plug. I got all my fluids ready to go as well however they are no fill lines, best approach I've read is to check inside the hull for each tank and fill them up?
    You mean fuel and 2 stroke oil right? If your MFD works, it will give you fill status of both, with a good deal of variation with respect to accuracy. The oil tank is best to fill visually. The fuel you will get used to over time.

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