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  1. #1

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    Polaris SLX 780 Problem only hitting 5700 RPM

    I have a Polaris SLX780 that I bought blown up.I rebuilt it over the winter including a new crank, pistons and boring, seals, gaskets. Cleaned up the ports a bit and also a carb kit. It starts fine and runs quite well but I can only hit 5700 rpm. It seems that under the right conditions it will rev up to 6700 rpm and feel even better. I have went through the fuel system and venting as well as the electrical system. All readings are very close to what the manual says. My only thoughts are that the CDI is intermittent or possibly an intermittent coil or plug cap. The only other thing I can see is that my MFD sits at 450 rpm even when the ski is off as well as my speedo only shows 25 kph when planed out at speed (I know its going faster than that!!) Has anyone heard of this or experienced it. I am new to the PWC world and these things are a little more complex than a sled or motorcycle so am having a heck of a time figuring this one out. Any help would be appreciated.
    Thanks


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    The MFD speedometer works via a pressure sensor inside the display. A clear plastic tube runs down to a pitot fitting directly under the drive shaft. Make sure the hose is not blocked, cracked or kinked. The plastic nipple under the drive shaft is fragile so if you need to shorten the tube and reconnect, be careful to not twist or yank on the nipple. A small zip tie is used to hold it in place.

    You can blow air or water through the tube from the end behind the display. It should pee out under the hull through a small hole in the leading edge of the pitot fin in the jet pump tunnel.

    The MFD always reading RPM suggests either the MFD is failing internally or there is electrical noise on the Yellow wire, which is where the RPM signal comes from.

    In rare cases the LR voltage regulator module is actually corrupting the Yellow wire signal (injecting noise) and confusing the MFD display.
    Last edited by K447; 08-04-2014 at 10:31 PM.

  3. #3
    BlueFishCrisis's Avatar
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    IT seems like a combination incorrect speed, incorrect RPM, and intermittent ignition issues may all point to the MFD failing internally as K mentioned. Unplug the MFD and run the ski. See if the running issue goes away. You wont be able to see RPM, but hopefully you have a good idea on the sound differences between 5700 and 6700 RPM....

  4. #4

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    Thanks, I will try it out and post the results

  5. #5

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    is there a fix for this LR regulating module injecting noise into the MFD? Can you put a capacitor in line to take teh noise to ground?

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by fitz288 View Post
    is there a fix for this LR regulating module injecting noise into the MFD?
    Can you put a capacitor in line to take the noise to ground?
    Often the LR module is actually not working properly anyways, in which case it needs replacement.

    I have not had enough LR modules go noisy and upset the MFD to have a definitive characterization of the possible work arounds. And if the bad LR module is not charging the battery then it gets replaced anyways.

    Capacitor on the Yellow wire is tricky since Yellow carries a high voltage AC signal.

  7. #7

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    Ok ....went to start it after sitting for a week and the battery was dead. A little strange so it maybe the LR is not allowing the battery to charge. I pulled the regulator since I saw some other people had done that...... and it acts the same. Right off the bat it seems to run fine and then bogs out again around 5700 RPM after 10 mins or so. went back over everything and even disconnected the MFD. Still the same results. I started going through the manual and found somewhere in troubleshooting that mentioned a clogged muffler for poor performance. There wasn't alot..... but but there was some water in there. I disconnected the coolant intake hose and hooked a garden hose up to it. with low pressure water comes out the actual exhaust like a tap. this tells me there might be a crack or hole in the jacket that surrounds the expansion chamber and may be the cause of my problems with water clogging the silencer. One more thing, if I plug the screen up near the manifold it stops pouring out. Can anyone confirm the operation of the cooling system.....I cant find much into about how it works but I don't think there should be water running out the actual exhaust where it meets the silencer.

  8. #8
    BlueFishCrisis's Avatar
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    These are a wet exhaust system. The intake comes from the jet pump, then travels to the front of the exhaust pipe and exhaust manifold simultaneously. Water is then injected into the exhaust pipe and out the exhaust into the waterbox. You will have water in the waterbox. The flow from the exhaust manifold continues through the cylinders and out the heads into the water rail. From the water rail, flow heads towards the rear of the ski where a t stat and pop off valve allow water to flow out the bottom of the water rail and finally out of the hull through the large nipple connected to the base of the exhaust outlet.

    I doubt your problem is a clogged exhaust system. I am betting your stator magneto is beginning to fail....

  9. #9

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    I took some readings from the stator and heres what I came up with:

    Red/purp to Yellow - 1.2ohms (.6ohms more than spec)
    Red/purp or yellow to Black - Open
    White /Yellow to Black - 214.7ohms (14.7ohms more than spec)
    Blue/Yellow to Red white - 70.1ohms (19.9ohms less than spec)
    Red/white to Black - Open
    Red/White to Green Red - 453ohms (37ohms less than spec)

    Just wondering if anyone has had similar readings and if putting a new stator on definitely fixed the problem. I took it out once this year and it was very calm with only a few waves. It seems like when the jet has the least resistance (going over a small wave) it kicks up to 6700 rpm and runs fine. I should also note that it seems like random cylinders are cooler after trying the ski out for a few mins and when I pull the plug out on that cylinder it looks a bit wet. this happens at random and can be any cylinder when I pull into shore. Kind of makes me think it might be the 20ohms difference on the pulser coil but not sure. Any input would be greatly appreciated and thanks for all the previous posts.

  10. #10

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    2 years in the making but finally got around to looking at this thing again. You were right ......it was 100% the stator.... replaced it and all is well.....after doing a little jetting it runs like a champ. Thanks for all your posts trying to help me figure this one out

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