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  1. #1

    How To Compression test on 951

    HI all,

    Can some one please run me through how to compresion test my 99 XPL, as ive borrowed a mates compresison test kit today.

    Or tell me where abouts in the shop manual it is as i cant find it

  2. #2
    KrunchovXPL-GTX-RX's Avatar
    Join Date
    Mar 2009
    Aiken, SC
    I do not think it is in the manual.

    No big deal. I will go over it for you along with some compression test folklore and mythbusting.

    The basics are such: Make sure you have a fully charged battery. Remove the plug from the cylinder you wish to start with (it does not matter which). Screw in the compression tester's fitting and make sure it is sealed in the plug hole (if you have the type that you hold up against the hole do not waste your time). Connect the tester and turn the engine over until the needle stops moving or the digital reading stops climbing. It only takes 4 to 5 rotations to max out (beyond that is wasting battery charge). Write down the reading (sure you can remember, but it does not hurt). Now switch the other cylinder.

    Folklore and myths: I leave the other plug in. Some say to take it out to lighten the load on the starter. IMO, you can do it either way, just do them both the same.

    Some say to hold the throttle wide open. Well, you have to on a DI but on carbed engines I have done them with and without WOT and there was absolutely NO difference in the readings.

    Some say to do it dead cold and some say hot. Again, you can do it either way but make sure it is either completely cold or at full operating temperature (one that is on its way to cooling off will be cooler in some spots than others and could cause a false variation in the reading).

    What should the reading be? Well, that depends on a couple of factors. The health of the starter and battery, for one, as well as the guage and temp and hours on the engine ect. You will see several numbers around for what a 947 should read. I have have had three SBT engines that all read 140, a stock original engine in an '02 GTX with 150 hours on it reads 150 and my own hand built engine in my '99 XPL with an ADA head and 50cc domes, six hole base gasket bored .25mm over pumps 155. All of these readings are on the same guage.

    The bottom line? If you are between 135-150 and the cylinders vary less than 8% then you are in good shape (some say 10% but I am picky with only two cylinders).

    If it is less than 135 and it is even then you would want to try charging the battery again, a different guage and or if the boat starts and runs well (turns up over 6800 with stock carbs, ect) and pulls well, then just ride it.

    Remember, you are looking more for a variation than you are a total number. Keep all the parameters the same for both cylinders and you should have accurate, reliable numbers to work with.

  3. #3

  4. #4
    So I have dont the test on a strong battery and got the following

    Front cylinder: 135 psi
    2nd Cylinder 130 psi

    Seems to be nice and healthy to me.


  5. #5
    KrunchovXPL-GTX-RX's Avatar
    Join Date
    Mar 2009
    Aiken, SC
    Not bad at all.

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