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  1. #1
    wille_tom's Avatar
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    Unhappy 1999 polaris pro 785 problem..

    Hello, i just got into jet skis this year, i ride dirtbikes and am mechanically able. i bought a pro 785 today, took it to the river and started it up, waited for the water to come out the 3 holes on the hull.. and took off.. it shot out and revved good, reached about 45-50 and bam! engine dies..
    went to start it and nothing.. waited about 5 seconds then it rolled over and started.. took off again.. and once again did the same thing.. took about 10 minuets of this repeated quiting untill it would just not go but a idle, would back fire, sounded more like just compression comeing out then backfiren though,,
    i took the carbs off and cleaned them through the diafram side. didnt take the pumpers off.. it did run better but will no plain out.. its doggy the whole time.
    i took the plugs out and the center plug looks pretty clean still.. outter ones are a little black. ill get some pictures off my cell phone if needed.

    oh, and when on the traailer the darn thing revs out awsome.. but drop it in and put the load of the water and shes junk


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome to the Hulk

    What model year is your Pro 785?
    Last two digits on the rear deck HIN plate.

    The Pro 785 is not a regular PWC. It is a race PWC, designed to compete in stock form, right off the showroom floor.

    The Pro 785 really can run very well, and strong, but it must be well maintained.

    If it is not in 100% perfect running condition, not only will it not run well, but you are likely to damage the engine.

    Is the machine stock, or has it been modified?

    Have you checked the cylinder compression?
    All spark plugs out, throttle held wide open...

    Unless you know the following things have been properly attended to, I would say at a minimum you need to remove and completely rebuilt those carbs. Use genuine OEM rebuild kits, not generic.

    Check the entire fuel system. If the gray Tempo fuel hoses are still on there, replace them all. If the oil hoses are original, replace them too.

    Install new fuel and oil filters, and clean and check the fuel/water separator, if it has one. Check for water in the fuel tank, and check the filler neck and cap for cracks. Make sure the cap seal is good.

    Also check the electrical box for moisture. Should be dry inside, with zero corrosion. Check that the correct CDI is installed.

    Make sure the spark plugs are the correct number.

  3. #3
    wille_tom's Avatar
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    well, i checked compression, i have 120 in both outter cylenders, and 0 on the center... tore the head off and sure enough the edge of the piston on the exhaust valve side is bad. and the cylinder is junk..

    anyone know of a company that would have parts and overnite shipping..
    thanks..
    tom

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by wille_tom View Post
    ...edge of the piston on the exhaust valve side is bad. and the cylinder is junk..

    anyone know of a company that would have parts and overnight shipping..
    Click here for a list of Polaris parts sources.

    For good used parts (and some new), email John Zigler.

    For new, you might try contacting Randy at Watcon

    Both are normally very responsive.

    The way most guys do it is to buy a good used cylinder, or have their own re-bored and Nicasil plated, then buy a piston and ring kit to match the cylinder bore.

    Have a look in the Polaris Classifieds - someone may have what you need.

  5. #5

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    The 785 cylinders can not be bored they have to be replated
    Contact Randy @ Watcon 1 360-692-3492

  6. #6
    wille_tom's Avatar
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    got a question.. whats the avrg. compression i should have on these cylinders?
    i did a test and got 120psi with plugs out and wot. the two outer ones of course.. just thinking since im already taken the pipes out should i just put new pistons n rings in all 3..
    also going to rebuild the carbs, and change all fuel/oil injector lines.. and clean the tank.

    thanks for the help guys

  7. #7

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    With a stock head, I would hope to see readings from 120-140 different gauges may present higher or lower readings. Altitude etc... may also cause differing readings. Higher elevation=lower compression.

    Being that both your Mag and Pto are the same, i'd guess your gauge reads a little low, and they are probably fine.

    However, since you are removing the C cylinder, i'd just pull the other two, and get a good look at the nicasil, pistons, rings etc.... If you see anything you don't like then consider what all parts you need to order.

    I'd also try to figure out what cause your piston failure, or it may just happen again! Was it water intrusion, foreign object, exceeded service limits, caught ring, lean, did crank let go,etc.... Post pix of the piston and such for us to see to help with opinions.

    Otherwise, sounds like you're on the right track. It's alot easier to figure a machine out, when you know everything is 100%

    And you are definitly right about removing the pipes, once the pipes are out, everything else is pretty quick to work with on the Pro.

  8. #8
    wille_tom's Avatar
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    redline, thanks for the info..
    the scoop on this ski is i bought it, 3 days ago (sat for a year and a half) the guy put 2 gallons fresh gas in it... well i didnt take it for a test run due too it being at his house and not close to water.. i took it home put her in the water and and made it about 1/4 mile and locked up.. anyhow i contacted randy from watcon and im going to send him all cylinders and pistons, so he can check it out and also hes the one who recommended this so he could point out the issue on what caused the initial cause.
    thanks to you guys i got hooked up with the right guy and advice. much appreciated.. and i will put some pictures up so i can get everyones advice.

    i did clean the carbs after the lockup, and i think this was the issue, center piston is melted one inch wide by 1/4 in a circle on the exhaust side. thus pointing to lean condition on a plugged carb, or oil failure im thinkn.

  9. #9

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    yep, i'd guess lean as well with a piston like that. that carb had to have been plugged somewhere or the diaphram all hardened.

  10. #10
    wille_tom's Avatar
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    ok, im thinking this was a oil issue, heres some pictures of the piston and the valves i pulled out... the center one is dry, unless thats cause it was running so hot.. the piston does have a hole in it on exhaust side on the piston skirt, will throw some pis of that later..
    either way im gunna rebuild the cards, and oil injection.

    i payed 2200 for this, its got the skatetrack stator, and shimplate.
    its pretty thick.. im assuming this gives more outta the hole?
    the guy is giving me a new piston a ring, and top end gaskets.
    .gave me the stock parts too. and the dolly cart.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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