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  1. #1

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    how much difference in cold compression test v. warm?

    Haven't had the chance to pull the battery out yet and assess it. Ski seemed to have ample oomph to turn over quickly but had to shut it down as too cold to run it on hose yet. Anyway, did a comp check witha cheap tester and only came up with 95+94lbs pto/mag. Can't get back to ski for couple of weeks as out of town but wondering if I have a problem. I pulled the raves and they were only slightly dirty cleaned em up with a rag alone! ant the cylinders / pistons were scuff free & looked real good. I can't explain this. My cousin rode the ski down in Florida the day before it came up to me and told me it ran fantastic and he should know as he is on these things continuously. It's weird as it starts right up for me and I would think if the compression was really that low the ski wouldn't even turn over. Thinkin I have to invest in a better comp tester and make sure the battry is freshly charged. Actron tester any suggestions?


  2. #2
    Danno135's Avatar
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    Fresh battery and better tester is what it sounds like to me. What kind of motor 787/951??

  3. #3
    flyin' the friendly skies airbornexp's Avatar
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    I think that he has a 2000 GTX 951 DI judging by his other thread about oxidation

  4. #4

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    Sorry Fellas, it's a 2001 gtx carb model

  5. #5

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    Someone reassure me here though please. Would a 951 carb model run wel if its compression was actually this low?

  6. #6

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    Jan 2008
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    Clermont FL.
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    compression

    I have a 02 GTXdi dd a compression check a while back. When it is cold it had 110 both cylinders. This was with a really old gauge. The reason I checked was the RPM is down and the speed is down about 10 mph at wide open throttle. But it idles perfectly and there is no miss even at wide open throttle. This the 951 engine with 160 hours.
    Last edited by divin10; 03-04-2009 at 08:42 PM. Reason: more info

  7. #7
    Danno135's Avatar
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    Ok, well it sounds like you need to buy a better quality tester. A 951s compression should be 135 psi at sea level. Thats OEM spec. You want your cylinders to be within 10% of each other and you have that, so next logical thing would be a retest with a quality tester. I believe the book tells you to warm up the engine to normal operating temp and then test. To be honest, you might have a 1-2psi decrease testing it cold. Don't know how anal you are it's your choice.

    To answer your question, I seriously doubt that engine would run very well at all with those kind of #'s.

  8. #8
    96XPSS's Avatar
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    Quote Originally Posted by ppkarnezis View Post
    Haven't had the chance to pull the battery out yet and assess it. Ski seemed to have ample oomph to turn over quickly but had to shut it down as too cold to run it on hose yet. Anyway, did a comp check witha cheap tester and only came up with 95+94lbs pto/mag. Can't get back to ski for couple of weeks as out of town but wondering if I have a problem. I pulled the raves and they were only slightly dirty cleaned em up with a rag alone! ant the cylinders / pistons were scuff free & looked real good. I can't explain this. My cousin rode the ski down in Florida the day before it came up to me and told me it ran fantastic and he should know as he is on these things continuously. It's weird as it starts right up for me and I would think if the compression was really that low the ski wouldn't even turn over. Thinkin I have to invest in a better comp tester and make sure the battry is freshly charged. Actron tester any suggestions?
    Something very cheap that everyone forgets to change is the schrader valve core within your compression tester. This is what leaks down. The seal is very small and when it gets brittle, it actually bypasses compressed gasses as it cannot hold all of it. So, before spending anymore on a new gauge, change the schrader first.

    And for added insurance against leakdown, install a tight sealing cap over the valve before each test.

  9. #9
    steach's Avatar
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    Quote Originally Posted by 96XPSS View Post
    And for added insurance against leakdown, install a tight sealing cap over the valve before each test.
    This is the ultimate assurance but my fingers hurt just thinking about testing my triple.

    My guess is a fully charged battery (12.2 volts +) will bring those #s up.
    One plug in and throt open.

  10. #10
    u887266's Avatar
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    Yes your compression will change when the engine is hot. On average all me ski's will gain about 15% when the engine is hot. I hope this will help you.

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