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  1. #1
    johnsonmtz's Avatar
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    The SLT780 LIVES!!!!

    Well, after some monstrous butt busting, I got the SLT780 back together in time to take on our trip to Minnesota this past week. I wasn't sure if I could get things done, but with the help of the Polaris gang on the 'Hulk I got it together and tested before I left.

    I set my low speed jets at factory 1 3/8 turns, but I gave all the high's an extra 1/4 turn to be sure I didn't lean out again. After a couple of hours of running I used a small flexible LED pen light like the one below to check piston wash.


    PTO and CEN had fairly large clean areas around the ports. The MAG had fingernails, so I gave that high speed another 1/8 turn out to richen it a bit to be on the safe side.

    Even after the top end job and running through the carbs (thoroughly this time ) the engine still wants to bog taking off from idle and it takes several blips of WOT to clear it out. It doesn't fall flat, so it doesn't seem to be a lean issue. It blubbers and smokes and then finally gets up and goes. Also, the ski is difficult to start and usually requires the throttle wide open to get it to fire.

    Tell me if this scenario sounds plausible. When checking piston wash, I noticed the top of the PTO piston was very wet. The CEN and MAG were both a dry, charcoal looking color in the middle of the piston. I'm wondering if possibly there is a leak in the fuel pump diaphragm and it's allowing excess fuel to enter the PTO crank case through the pulse line I've also noticed that it loads up very bad if I set with the engine off and the fuel valve set to ON. It doesn't happen has bad when I turn off the fuel when the ski is just setting. This seems that fuel is some way getting constantly dumped into the engine somewhere.

    To test this I plan to remove all lines from the pump, put a hand vacuum pump on the pulse nipple, and give it a couple of inches of vacuum. My guess is if the pump is good it should hold a vacuum indefinitely. If it's bad (small hole or tear) it should not be able to hold much, if any vacuum. Does this sound like a reasonable guess and testing method?

    If the diaphragm is bad, it looks like Partsland.com has a pretty good price on a gasket/diaphragm kit. http://partsland.com/index.cgi?N=121...-780-FUEL-PUMP

    If I'm barking up the wrong tree please let me know. The ski is running good now except for the hard start and bog issue. I'm convinced the two are related and I'd like to track this down and finally put it to rest.

    Thanks for all the continued help.

    KJ


  2. #2
    johnsonmtz's Avatar
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    *** bump ***

    Anyone have any thoughts on this idle flooding issue

    Does a leak in the fuel pump idea sound possible??

    KJ

  3. #3
    BBCaprice's Avatar
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    Dont be afraid to turn the lo-speeds in a little as this can be causing your issues. Go 1/8 at a time in and see if it clears up.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by BBCaprice View Post
    Dont be afraid to turn the lo-speeds in a little as this can be causing your issues. Go 1/8 at a time in and see if it clears up.
    Try that for the bog problem. ^^^^

    As for the flooding, it's very possible.

    As pressure builds up inside the fuel tank from the sun beating on it, it will push the fuel through the lines if the fuel selector isn't off.

    You may need a new pressure release valve. They are supposed to open at 1.5 psi. If yours is stuck closed, the pressurized fuel will go to the carbs.

  5. #5
    johnsonmtz's Avatar
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    Quote Originally Posted by xlint89 View Post
    Try that for the bog problem. ^^^^

    As for the flooding, it's very possible.

    As pressure builds up inside the fuel tank from the sun beating on it, it will push the fuel through the lines if the fuel selector isn't off.

    You may need a new pressure release valve. They are supposed to open at 1.5 psi. If yours is stuck closed, the pressurized fuel will go to the carbs.
    So the tank is suppose to have very little positive pressure? Hmmm, I have noticed some times when I remove the fuel fill cap that there is some pressure in the tank. I'll give that a look.


    I have all the low speed needles set the same. Why would the PTO piston look so wet and CEN and MAG look dry? I'll check the spark on that cylinder in case that's the problem, but I don't seem to feel any kind of miss and the there is plenty of power once the engine "clears out".


    KJ

  6. #6
    sounds to me like you have a leaky needle and seat in the pto carb ,
    that would explain all of the symptoms and they clear up at higher rpms because the fuel goes to use

  7. #7
    johnsonmtz's Avatar
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    Quote Originally Posted by ZMANN View Post
    sounds to me like you have a leaky needle and seat in the pto carb ,
    that would explain all of the symptoms and they clear up at higher rpms because the fuel goes to use
    Nope. New needles and seats on all 3 carbs and the pop-off is exactly 23PSI on all of them.


    I previously thought the same thing and last year I purchased new needles and seats for all 3 carbs. I never got around to installing them until a couple of weeks ago when I got the engine back together. With such even pop-off pressures I thought I could rule out bad needles as a possibility.


    Correct me if I'm wrong, but as I understand these SBN carbs there is no idle bypass circuit right, the fuel must always go through the low speed needle first......right? Also, the fuel passes directly from the internal filter to the needle valve, then on to the low and high speed valves/jets. So, if my pop-off pressures are right where they should be and that is the only way fuel comes in, how could excess fuel be getting in the PTO?


    I hope tonight to perform my vacuum check on the fuel pump. At minimum I'll be able to rule that out as a possibility.....or I will have found the problem. Unless there is something I'm missing, the fuel pump seems to be the most likely culprit.


    KJ

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Is it possible the needle/seat O ring is damaged?

    How about a piece of debris holding the needle from seating fully?

    How are the reeds? if they're damaged, then fuel will flow into that cyl.

  9. #9
    johnsonmtz's Avatar
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    Quote Originally Posted by xlint89 View Post
    Is it possible the needle/seat O ring is damaged?

    How about a piece of debris holding the needle from seating fully?

    How are the reeds? if they're damaged, then fuel will flow into that cyl.
    I'm hesitant to think there is a problem with the needles and seats since this problem existed before I did the top end. I could test the pop-off pressures again, but I really doubt there is a problem there.


    I installed the reeds 2 years ago. They are Boyesen Power Reeds that I bought new. Unless something major has happened there I don't think that would be the problem.


    Basically I have been chasing this problem ever since I bought the ski in 2004 . So, something just isn't quite right and I can't seem to nail it. At this point, the only item I have NOT touched is the fuel pump.

  10. #10
    you can see a leaky needle issue by watching the throat of the carbs for fuel dumping

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