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  1. #1

    Polaris SLT 780 idle issue & water separator

    Been using this site for some time, getting as much information as I can for my 96 SLT 780. This is my first post and Iím in need of some help. As stated I have an SLT 780. I picked up for 200 bucks with trailer but needed a new motor. I got a used one off eBay and freshened it up a bit. I have replaced spark plugs, battery, fuel lines, selector, rebuilt carb and and fuel pump. Removed fuel tank, cleaned tank & sending unit. Removed jet pump and cleaned/inspected. Set carbs to factory spec for 96 780, and pop off is at 20psi.

    The problem Iím having is, this thing will fire right up and idle great on trailer even on hose. But once in water it starts fine but motor shakes like a bad misfire? I know a load is being put on the engine which is the reason for the change. My main concern is that the water separator seems to be what I would consider too low? When running the fuel level drops to the point the fuel filter inside barely has fuel up to the bottom of it? After sitting it fills back up to about 3/4 full. Is this normal or should this thing be completely full? I originally installed the clear blue fuel lines but later read that they arenít highly recommended so I swapped that out for some regular reinforced fuel line and havenít tested it back in the water since. Could the blue fuel line have had anything to do with this issue.

    I donít believe the bad idle is due to water intrusion because it would run poorly on the hose too correct. Or am I mistaken there? Mfd displays at 3050 rpm at idle on trailer which from what I read is about right. Not sure what my next step should be. Any advice would be appreciated!


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk.

    You've done some reading, that's good.

    Yes, get rid of the clear fuel lines. It's too soft for use in front of the fuel pump. The pump creates vacuum and needs to pull the fuel into it. (fuel tank pressure also helps to push fuel to the pump as well)

    The fuel/water seperator should be full. if it's not, there is a problem.

    1. make sure the fuel tank check valves are working properly. One lets air into the fuel tank, but not out. And the other lets excess air pressure out. (about 2 PSI I believe) As a test, you can unscrew the fuel cap and see if that helps to keep the seperator full.

    2. Make sure the fuel lines are on the correct fitting of the sender. (its marked with abbreviations)

    3. Make sure you have good compression on the PTO cylinder as that is what drives the fuel pump.

    4. make sure you have air tight connections on all your fuel hose fittings

  3. #3
    Click avatar for tech links/info K447's Avatar
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    Welcome.

    Look for places air might be leaking into the fuel feed hoses. The fuel pump creates suction to draw fuel from the tank. If there is an air leak anywhere between fuel tank and fuel pump, the system will not be able to pump fuel effectively.

    Since the air is appearing inside the separator, check every seal and hose connection, starting with the water separator itself. There are two o-rings in the separator. A big o-ring seals the Ďlidí and a much smaller one seals the internal metal fuel filter, if your model has that filter.

    If the lid is not sealing properly air will be pulled into the jar.

    If the separator is all good follow the fuel supply hose towards the fuel tank. The original fuel selector valves are known to die from old age, and the rubber seals inside harden and leak air. Replace with a new fuel selector valve.

    Make sure the hose clamps are doing their jobs. Over tightened gear clamps can crush or crease the Fuel hose and create air leaks.

    At the fuel tank, sometimes the top fittings into the sender can work loose and develop air leaks.

    As already mentioned, the rim of the sender cap has tiny markings for the fittings. Make sure you are not trying to use a fuel return or tank vent fitting as fuel supply.

  4. #4
    Thank you for the reply’s! I will leave the gas cap off tonight and see if there is a change. If it fills up over night what would that indicate? The compression is good across the 3 cylinders about 130psi throttle pulled open. Sending unit was removed and I saw the markings for the nipples and they are hooked up correctly (my sending unit has no reserve?) I have not replaced the o rings in the separator as they seemed decent but will go ahead and swap anyway! I saw ur post on the rebuild of it! I’m using the reeds out of the old engine that had damage and the PTO reeds are OK but not great. Would this hinder the pull from the tank? Will double check all hose clamp connections as well!

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Sorry, I meant try removing the gas cap while running on the garden hose and see if the water separator fills up fully.

    You do not have reserve. You have a low fuel indicator and what we call "limp mode". This limits your engine RPM to around 4300 in order to conserve fuel allowing you to get back to refuel.

    Your main fuel line should be on the RES of the fuel sender. Vent hose to the VENT. And return hose going to the last fitting.

    Weak reeds may hinder fuel pump operation. The fuel pump works by vacuum moving the diaphragm one direction, and positive crankcase pressure pushing the diaphragm other direction. This happens for every revolution of the crankshaft. So if your reeds aren't sealing well, it can have a negative effect.

  6. #6
    No worries that makes sense! So I started with the easiest thing first. I picked up a couple o-rings for the separator, filled it up with gas and fingers crossed but it seems to be staying full while running! Ran it two separate times on the hose last night and the fuel level hardly dropped. Before replacing the orings I opened the tank and did get a whoosh of air so I assume the check valve are good and tank is building pressure. Only other thing I did was double check all hose clamps like stated above, went ahead and loosened a few and re righted just to ensure none were too tight. Going to get it back in the water this weekend and we’ll see how she does! Thanks again for y’alls help

  7. #7
    Well was hoping it was fixed but no luck. Checked on the ski last night and noticed the fuel level in the separator was down about an 1/8” than it was last time. So I fired it up just to make sure everything was still okay and sure enough the fuel level in the separator drops right down to the bottom of the filter again? Going to try and run it with the fuel cap off tonight as suggested as see if it does anything. Also thinking about pulling the reeds and swapping the ok PTO reeds with one of the other cylinders reeds that look perfect. Unsure about the check valves but will pull them out tonight aswell and inspect.

    Also fuel tank be able to hold pressure for couple days if nothing is opened (gas cap, and fuel selector is shut off)?

  8. #8
    Click avatar for tech links/info K447's Avatar
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    Confirm that the fuel in and fuel out hoses are on the correct fittings on the fuel:water separator.

  9. #9
    The are but I will definitely double check that aswell tonight. My separator is above the gas tank so i had the whole thing out when I pulled the tank and there’s arrows indicating flow on the cast lid

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    When you unscrew the gas cap, is there a pressure build up inside? (gas cap slightly blows off)

    Try the reeds swap please

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