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  1. #1

    Checking compression

    How do you check the compression? Do you check 1 cylender at a time with the spark plugs in the other cylenders or do you remove all the spark plugs and check. Had someone tell me that 110 when all the plugs were removed on a 2000 Genesis was low and bad. My understanding is that this is good. I was getting in the 120-range by checking one cylender at a time.


  2. #2
    1996 SL900 & SL700 turk0004's Avatar
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    I checked mine with the plugs out and got 120 across the board after a complete engine rebuild, but that was on my SL900, not sure about a Genesis.

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    casey67's Avatar
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    Check compression with all plugs removed,good battery,throttle wide open. I'm not sure of exact # for your Genisis.Most guages read different,I got 20 PSI difference with 2 different adaptors,same guage.In my opinion 110 psi even accross all 3 cylinders is OK.Try another guage for giggles.

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    Water Bum Rodneyae's Avatar
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    90-95 psi is min. for compression. So 110-120 are good numbers. The higher the better and more power you will have. compression will be lower on a fresh engine untill the rings seat in to the cyclinders.

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    make sure you stick the spark plug back in the socket and have it laying on a grounded un painted peice of metal so the spark can release its energy , if you dont you might risk blowing your coils or :edit or dont have the lanyard in durr

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Arrow Checking engine compression

    Quote Originally Posted by zakos View Post
    How do you check the compression?
    Do you check 1 cylinder at a time with the spark plugs in the other cylinders or do you remove all the spark plugs and check.
    Remove all the plugs, and ground the wires.
    Had someone tell me that 110 when all the plugs were removed on a 2000 Genesis was low and bad. My understanding is that this is good.
    I was getting in the 120-range by checking one cylinder at a time.
    Quote Originally Posted by casey67 View Post
    Check compression with all plugs removed, good battery, throttle wide open. I'm not sure of exact # for your Genisis. Most guages read different. I got 20 PSI difference with 2 different adaptors,same guage. In my opinion 110 psi even across all 3 cylinders is OK. Try another guage for giggles.
    If you have a Polaris with a domestic (red) motor, there is often a grounded fitting on top of the motor that you can clip each of the spark plug boots onto. This prevents the ignition coils from attempting to discharge into the air at the end of each plug wire, and safely routes the spark energy to ground.

    On the older blue Fuji motors, put spark plugs into the wire boots, and clamp the plugs against something bare metal on the motor (or find and install one of the plug wire grounding brackets from a domestic motor).

    Buy and use a good compression gauge, and always use the same gauge, to eliminate the possibility that a different gauge will produce different readings. Take care of the gauge, don't drop it or bang it around.

    Your engine should crank with the stop lanyard out, which means there will be no spark (which is a good thing). Hold the throttle open, and crank until the gauge reading settles down. This should only take a few seconds.

    If your battery is getting old, or is not fully charged, you will get low, and less consistent, compression readings.

    You do not want to crank until the nth degree, you just need enough cranking time (a few seconds) to get a consistent reading on each cylinder. The first few times you test compression, go through and test all the cylinders twice. The numbers for each cylinder should be fairly close.

    Consistent readings from cylinder to cylinder are usually more important than the absolute pressures. You want the variation from cylinder to cylinder to be 10% or less, and most mechanics would prefer readings that vary by less than 5% between all the cylinders.

    Do understand that compression testing can only give a partial picture of the internal health of an engine. The motor is not running at normal running RPM, so the pressures shown on the gauge are less than what is happening under operating conditions.

    If the cylinders are worn, or the piston rings are not sealing really well in the cylinders, the readings you get will be lower than normal. Depending on the condition of the motor, these lower readings can mean the engine needs rebuilding, or it is just wearing normally, and has life left in it.

    A significant variation in compression readings for one or two cylinders usually means something is going wrong, and further investigation is required. Do not use the boat until you understand why the compression is so different in these cylinders.

    2-stoke motors are intolerant of out-of-spec operating conditions. If your fuel-air (carbs, air leaks) mixture is wrong, or the lubricating oil feed is not working, it can cause deterioration inside one or more cylinders.

    Catching the problem early, because you have been checking compression every so often (and have been paying attention to any unexpected changes in the motor's overall operation and power output), can reduce the amount of damage, and the cost of repair, significantly.
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  7. #7
    Quote Originally Posted by K447 View Post
    If you have a Polaris with a domestic (red) motor, there is often a grounded fitting on top of the motor that you can clip each of the spark plug boots onto. This prevents the ignition coils from attempting to discharge into the air at the end of each plug wire, and safely routes the spark energy to ground.
    2001 Genesis with no grounding plate. How should I properly ground the boots?

  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by K447 View Post
    ... Your engine should crank with the stop lanyard out, which means there will be no spark (which is a good thing). Hold the throttle open, and crank until the gauge reading settles down. This should only take a few seconds. .
    Quote Originally Posted by Number1HoustonTexansFan View Post
    2001 Genesis with no grounding plate.
    How should I properly ground the boots?
    If you leave the lanyard out, there is no need to ground the boots, as long as the engine will crank with the lanyard out.

  9. #9
    Quote Originally Posted by Number1HoustonTexansFan View Post
    2001 Genesis with no grounding plate. How should I properly ground the boots?
    I leave the plug in the boot and then use a wire with alligator clips to ground the plug to the engine.

  10. #10
    Bernie's Avatar
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    Use a bridge on the starter solenoid and you can crank it with the lanyard out and not have to worry about spark issues at all

    Bernie

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