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  1. #1
    Tony's Avatar
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    780 COMPRESSION #'s

    Looking at a '96 SLT 780.

    Front cyl. 125-130 psi
    middle cyl 105-110 psi
    rear cyl 115-120 psi


    What about the center cylinder? to low? any thoughts?


  2. #2
    ph2ocraft's Avatar
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    A little off but that could be gauge related. Were these numbers with a hot or cold engine, did you crank the engine until the gauge no longer went up or just a few revelutions of the engine?
    Was the battery completely charged and rotating the engine freely?
    Can you look into the cylinder with a flexible light and inspect the cylinder walls?
    The main thing is the cylinders all be within 10% of one another.
    Oh and WELCOME to the Green Hulk Forums!!

  3. #3
    Tony's Avatar
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    THANKS FOR THE WELCOME

    Quote Originally Posted by ph2ocraft View Post
    Were these numbers with a hot or cold engine,
    Ran on hose for 3-5 minutes

    Quote Originally Posted by ph2ocraft View Post
    did you crank the engine until the gauge no longer went up or just a few revelutions of the engine?
    Somewhere in between... about 2 seconds worth at WOT. If spun until guage stopped I might have been able to squeeze a few more pounds out.

    Quote Originally Posted by ph2ocraft View Post
    Was the battery completely charged and rotating the engine freely?
    Yes engine spun freely with strong battery.

    Quote Originally Posted by ph2ocraft View Post
    Can you look into the cylinder with a flexible light and inspect the cylinder walls?
    The main thing is the cylinders all be within 10% of one another.
    Oh and WELCOME to the Green Hulk Forums!!
    Couldn't look inside. #'s are close to 10% but not quite there. Is there a reason the center one would be lower than the other two? Is this an eminent problem? Would you buy a machine with these #'s?.

  4. #4
    Tony's Avatar
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    Well I decided to take a chance and bought the ski last Tuesday. Ran it for about 4 hours on the lake. Seems to be running good and strong.

  5. #5
    ph2ocraft's Avatar
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    This got past me while I was on vacation. Great ride in my book,
    Cut 1/8 inch off of the spark plug wire and screw the cap back in.
    Make sure fuel lines are not kinked over.
    How about some photo's??

  6. #6
    What gauge did you use to test the compression. I need to do this also.

    Thanks

  7. #7
    Tony's Avatar
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    Quote Originally Posted by ph2ocraft View Post
    Great ride in my book,
    Really? I must admit that between the day I bought it and the first time I rode it I was having second doubts. Correct me if I'm wrong but didn't the model get discontinued after 3 years? And I see alot of info on other engines but the 780 seems like it may have had a short production run as well. Don't get me wrong, after riding it I really like it, but I was/am under the impression it is one of the less popular models.

    I was so excited to ride it last weekend ( I hadn't even ridden one before) that I left the camera and GPS at home. I'll get some of it on the water next weekend.

    Quote Originally Posted by rickreeves1 View Post
    What gauge did you use to test the compression. I need to do this also.

    Thanks
    Just a standard compression guage. I'm an auto tech and don't remember which of the two I used. It was either a Cornwell or MAC

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    They didn't abandon it completely... it got a little bit more stroke, triple pipes and some exhaust valves. Then it became the 145 HP PRO 785 which was built until 2000? Polaris just opted to go with the bigger Domestic engines (900, 1050, and 1200) rather than the Fujis for production models.

  9. #9
    ph2ocraft's Avatar
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    Not only that, the basic design started as the 650, then the 750 and of course the 780. The bugs were worked out for the most part and the 780 power plant has proven to be extremely reliable.
    A little maintenance and they run forever.

  10. #10
    BBCaprice's Avatar
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    Quote Originally Posted by ph2ocraft View Post
    This got past me while I was on vacation. Great ride in my book,
    Cut 1/8 inch off of the spark plug wire and screw the cap back in.
    Make sure fuel lines are not kinked over.
    How about some photo's??
    ph2-when you say cut 1/8 off plug wire,which end are you talking about? At the spark plug itself or in the box?

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