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  1. #1
    abarrassr07's Avatar
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    should i redo top end on 951?

    well i did a compression check on the rx, it has exactly 120 psi on each cylinder. Am i in danger of blowing an engine? should i do a top end now or will i be ok for a while. forgot to hold the throttle wide open while checking. does this make a difference?


  2. #2
    My Jet Ski drinks more than yours Glenozzy's Avatar
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    sounds fine??? why did you check something wrong?

  3. #3
    Rebcammaz's Avatar
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    sounds good to me too, hoding the throttle open does not make a difference when checking the compression of the engine, but how many hours on the engine approx.??

  4. #4
    How do you know that guage is right?

  5. #5
    mibunkerking's Avatar
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    i noticed a good bit of difference when i forgot to open the throttle. remembered when i switched cylinders, retested and noticed almost a 20+/-psi difference on a brand new tester. you might want to retest it to be sure.

  6. #6
    BLACK PEARL's Avatar
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    new rebuild

    Quote Originally Posted by mibunkerking View Post
    i noticed a good bit of difference when i forgot to open the throttle. remembered when i switched cylinders, retested and noticed almost a 20+/-psi difference on a brand new tester. you might want to retest it to be sure.
    i just rebuilt a 951 di and i broke it in and it only tested 110 lb. wtf. just read this thread and i didn't hold the throtel open. and retested and the eng has 135.lb thanks for this info.

  7. #7

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    warm the engine on the hose for about a minute.

    now test the comp while having both plugs out andthe throttle pulled.......

    TLR

  8. #8
    abarrassr07's Avatar
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    Quote Originally Posted by TLR View Post
    warm the engine on the hose for about a minute.

    now test the comp while having both plugs out andthe throttle pulled.......

    TLR
    got you, nope nothing wrong at all, the dealer was trying to sell him self a $700 rebuild by saying stock is 155 and 120 is way too low and needs a rebuild, which he wouldnt benefit from anyway bc i do the work myself. thanks for the replies, im going test again tomorrow without the plugs.

  9. #9
    KrunchovXPL-GTX-RX's Avatar
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    Holding the throttle wide open matters on a DI because it puts the MPEM in "Flood" mode and shuts off the injectors.

    I have done it both ways, throttle open, throttle closed with different guages back to back and the readings did not change on a carbed 951.

    That said, the actual amount of compression is not as important as the balance. They should be within ten percent of each other.

    The issue of stock compression on a 951 is rather a touchy one but I have seen it stated as between 145-155 several times. I have rebuilt a number of them that pumped 150ish right from the get go and picked up a few as they broke in. I have had a couple of aftermarket remans (cough, sbt, cough) that pushed 140 (wich they claim is stock). Others say 135 is stock comp on a 951.

    So, is 120 low? Maybe. Assuming, as Piranah pointed out, the guage is correct. And stock comp really is in the 150s (at your altitude, that matters too).

    Try what TLR suggested but let me suggest a good long low amp (say 2 or less) charge on the battery as well. At least a good six hours.

    You may also want to borrow a buddy's guage and see what it reads your engine. If you have the rubber tipped type where you hold it up to the spark plug hole you need to throw it away and get one that screws in (analog or digital is up to you). The press in guages will almost always read low unless you are The Hulk or Barry Bonds.

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