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  1. #1

    Excess black sludge spitting out and not revving over 5000rpm

    1995 Polaris SLX 780

    I had a previous thread for the not revving issue which I still have not solved. I've done a partial carb rebuild (not needles, seats, jets), replaced fuel pump, cdi, ignition coils, still won't rev at full throttle. So yesterday I was toying with it, and it made a loud POP, and then my house and everything around it was covered in tons of black sludge. I've read that this is a sign of needing needles and seats and that it is excessive fuel dumping out of the exhaust. Any ideas? Click image for larger version. 

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  2. #2
    casey67's Avatar
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    When there is way too much fuel in the mixture, but mostly caused by lack of spark- yep unburnt fuel in the exhaust and because gas is a solvent, it loosens carbon/oil residue that coated the exhaust.Probably alot worse on pre-mix.

    5000 rpm is about where a 3cyl running on 2 cylinders will rev.
    When there is a lack of spark-raw gas in exhaust- then spark comes back, it will cause a pop in exhaust ( more of an explosion )

  3. #3
    Quote Originally Posted by casey67 View Post
    When there is way too much fuel in the mixture, but mostly caused by lack of spark- yep unburnt fuel in the exhaust and because gas is a solvent, it loosens carbon/oil residue that coated the exhaust.Probably alot worse on pre-mix.

    5000 rpm is about where a 3cyl running on 2 cylinders will rev.
    When there is a lack of spark-raw gas in exhaust- then spark comes back, it will cause a pop in exhaust ( more of an explosion )
    Ugh. Back to the search for the reason I'm not getting spark. This is driving me nuts. Thanks for the reply, I appreciate the help.

  4. #4

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    wow...never seen this before. once you figure out the cause it looks like you should take the whole exhaust apart and clean it. If it's doing that I can just imagine how much is in the waterbox...a bubblin crude! In reference to the show "The Beverly Hillbillies"...yes I'm showing my age!

  5. #5
    casey67's Avatar
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    Yeah, if it wasn't 25 years old I would have been more concerned. Ive seen a few almost as bad. The ski's that have the exhaust exit dirrectly at the jet pump really make a mess of the jet pump and ride plate after 5-10 years without a power wash.

    LOL.. they still show " Beverly Hillbillies" reruns. Im sure plenty of young stoners have watched a bunch of those reruns.
    Now if your talking about the "first run" of those shows- that would show your age.

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I'm going to agree with Casey. Do yourself a favor and buy 3 of those cheap in-line spark testers that attaches between the plug wire and the spark plug. They will show you which cylinder is not firing and when.

    I've had magnetos failing when it gets warm, but shows spark at idle and/or cold engine.

    And consider doing the needles and seats along with pop off pressure testing when complete. You can possibly have fuel dripping out of the carb if pressure is too low.

  7. #7
    Ok, I installed 3 in-line spark testers and now I'm even more confused. All 3 cylinders had spark and it stayed consistent throughout about 10 minutes of idles and revs. Now what?! Lol.

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Remove the flame arrestor and observe the carbs while the engine is running.

    Do you see any fuel dripping from the middle of the carbs? (leaking needle and seat)

    Do you see a bunch of fuel spitting out the top of a carb? Possible broken reed valve.

    Other possibility might be the magneto failing at higher RPM. Will need to leave the spark testers installed while riding with the seat off to observe, or while ski is strapped to the trailer in the water.

  9. #9
    Quote Originally Posted by xlint89 View Post
    Remove the flame arrestor and observe the carbs while the engine is running.

    Do you see any fuel dripping from the middle of the carbs? (leaking needle and seat)

    Do you see a bunch of fuel spitting out the top of a carb? Possible broken reed valve.

    Other possibility might be the magneto failing at higher RPM. Will need to leave the spark testers installed while riding with the seat off to observe, or while ski is strapped to the trailer in the water.
    I'll take them off and observe today. I haven't noticed fuel spitting out of the top of the carbs and I had them off a couple weeks ago and I inspected reeds and they seemed fine. I will hope I can see fuel dripping from the middle of the carb so I can do the easy repair and be done. I appreciate the help with this, my other thread died and this new sludge stuff was a new issue. I'm trying my best to get this resolved without needing a mechanic but this has been a tough one!

  10. #10
    Quote Originally Posted by xlint89 View Post
    Remove the flame arrestor and observe the carbs while the engine is running.

    Do you see any fuel dripping from the middle of the carbs? (leaking needle and seat)

    Do you see a bunch of fuel spitting out the top of a carb? Possible broken reed valve.

    Other possibility might be the magneto failing at higher RPM. Will need to leave the spark testers installed while riding with the seat off to observe, or while ski is strapped to the trailer in the water.
    Well I took a close look, what I found was that the PTO cylinder has a noticeable amount of fuel spraying out above the butterfly. I did not notice the other two cylinders showing this. There's clearly a lot more fuel dumping into the PTO cylinder. Needle and seats?

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