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  1. #1
    thetolleyman's Avatar
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    Pressure test, how exactly?

    Picked up a compression gauge and tried to test today. I get 125 +/- when all plugs removed and turning over with starter. I get 175+/- when all plugs in and running. Am I doing it wrong? This is an XLT1200.


  2. #2
    THE PLATE MAN JIM'S PERFORMANCE's Avatar
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    Quote Originally Posted by thetolleyman View Post
    Picked up a compression gauge and tried to test today. I get 125 +/- when all plugs removed and turning over with starter. I get 175+/- when all plugs in and running. Am I doing it wrong? This is an XLT1200.
    125 is correct you do not check compression with engine running.

  3. #3
    My understanding is: All plugs out, plug wires safely laid off to the side, throttle wide open with a bungie or rope to hold it full down, compression gauge in one plug hole at a time, crank for about 5 seconds and log result. Rinse and repeat. Lots of youtube videos on this out there as well. Search for waverunner compression test.

  4. #4
    xray328's Avatar
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    How did you check the compression with it running???

  5. #5

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    Its easy to check with engine running, if you don't take the plug wires off the other 2 plugs the engine may start. HOWEVER, that is not an accurate reading for comparision. You should only have one plug out at a time, if you have all the plugs out the engine will not crank the same speed and you will have gas mixture coming out over the top of the engine. You should have all the plug wires grounded though to prevent potential problems. I use 3 old plugs and put them on the wires, then place the plugs down by the power valve covers. That way the spark cannot ignite the mixture coming out of the plug hole if you don't have the tester in it. Hold the throttle wide open and crank a few seconds to get your reading. put a plug in that hole and do the same test with the others.

  6. #6
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    ^^^ huh?

  7. #7

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    [QUOTE=LT1GMC;1993566]Its easy to check with engine running, if you don't take the plug wires off the other 2 plugs the engine may start. HOWEVER, that is not an accurate reading for comparision. You should only have one plug out at a time, if you have all the plugs out the engine will not crank the same speed and you will have gas mixture coming out over the top of the engine. You should have all the plug wires grounded though to prevent potential problems. I use 3 old plugs and put them on the wires, then place the plugs down by the power valve covers. That way the spark cannot ignite the mixture coming out of the plug hole if you don't have the tester in it. Hold the throttle wide open and crank a few seconds to get your reading. put a plug in that hole and do the same test with the others.[/QUOTE

    +Huh

  8. #8

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    From GroupK.com:
    THE ENGINE - 1) All accurate readings must be taken from a dead cold engine. A warm engine will yield slightly lower numbers. 2) The exhaust pipe and carburetor must be installed. The restrictions in the carb throat and the back pressures of the exhaust system can affect the readings. 3) Have a full charge on the battery. The speed that the engine is spun, has a significant affect on the indicated reading. Having the charger connected during the test insures maximum starter motor RPM's. 4) Leave a spark plug in the cylinder not being measured. Contrary to what you may think, the engine will turn over slightly faster with the opposing spark plug installed.

    THE TEST - 1) Make sure both spark plug caps have spark plugs mounted in them, and those plugs are grounded to the cylinder or head. 2) Hold the throttle wide open to admit the maximum amount of air. 3) Hold the start button down until the needle on the compression gauge is no longer rising. 4) Test both cylinders.

    DO NOT squirt any oil into the cylinders to improve ring sealing for the test. The presence of added oil can cause readings 20 - 30 psi above the accurate "normal oil presence" reading".


    From me:
    It is not good for the electronics to crank the engine over without the spark plug wires on a plug, the loose wire causes the voltage to go very high trying to spark and can damage the cable or coil. Thats why its best to have a plug in the wire, and let the plug lay against the side of the engine.
    If you crank the engine with the other 2 plugs in and the wires on them, it can start while you are trying to do your test, not good.

  9. #9
    A child of five would understand this. Go get one. pierowman's Avatar
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    Quote Originally Posted by LT1GMC View Post
    From me:
    It is not good for the electronics to crank the engine over without the spark plug wires on a plug, the loose wire causes the voltage to go very high trying to spark and can damage the cable or coil. Thats why its best to have a plug in the wire, and let the plug lay against the side of the engine.
    If you crank the engine with the other 2 plugs in and the wires on them, it can start while you are trying to do your test, not good.
    This is not entirely acurate. The reason....when theres no completed circuit no damage can be done.

    Simple example.
    Say you have a light switch and a light. Theres two wires hooking the two up. While connected turing the switch on lights the light bulb and turning the switch off obviously light off. Now disconnect a wire from the bulb and turn the switch on. The bulb doesnt light, the switch has power, and no damage is done. The reason is the circuit is not completed so no power runs through the wire.

    What can happen how ever is if you put the cable say directly to ground no spark plug. What happens is a direct short to ground which this can do damage to electronics and melt the insulation around the sparkplug wire. Simply laying a properly insulated cable down should be fine but if the insulation is damaged in some way unknownly you could be setting the stage for disaster. A safe thing to do is lay them on somethign non-conductive like a board or the fiberglass hull. I dont like the idea of having the spark plugs in the cable because you can really be hurt by that and it doesnt really prevent frying the electornics if there is a problem with the insulation of the cable because electricity always goes the less restrictive route.

  10. #10
    Yamaha artisan Cutlass's Avatar
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    I always disable coils or directly ground the plug wires for compression tests (cars, boats, PWCs, ATVs). I just don't want an open sparks near any possible fuel fumes or stray gas. I've seen a fire started that way once.
    I've also performed a running compression test before, but its not a common test and usually doesn't serve much purpose.

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