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95 SLX 780 Engine Problem

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  • 95 SLX 780 Engine Problem

    I ran a compression test and the cylinders read as follows. #1 120psi #2 120psi and the #3 60psi. Here is how the problem started. On Sunday Jusly 27th. Tried starting it before I went out to the lake but it would not start. It would crank but get stuck. I pulled the spark plugs and all three cylinders were full of fuel. I live in Dallas and its been really hot here. I had left the fuel on all week and the cap was on tight so it could not vent the preassure. Celared out all the fuel and it started. Took it out and ran great all day untill the end. It just died. Fuel begins to fill up into the #3 cylinder first and then goes to the others. There are some metal shavings that come out only on the #3 cylinder. Any Ideas on what it may be.

  • #2
    Re: 95 SLX 780 Engine Problem

    Here is what you may have. I know these ski's very well. First of all your front piston has a history of not getting enough fuel. They make a fuel pump add on that makes sure you get enough gas. Now when you don't that piston fires early sometimes and blows a hole in your piston. That may be the cause. Now the result. Metal pieces fly around inside that cylinder and break up more pieces and you will hear a rattling. At this point you need to remove the cylinder and rebuild it. might as well check your other pistons. Now when you are done with this engine. Here are some things i have done. Installed a extra fuel pump. Completely remove your thermostat and do not install another one. I don't care what anyone says. Those year of polaris's with those blue engines will run too hot. You need all the help you can get to get them run cool. The last thing is remove the oil pump and install the 15.00 block of kit and use pre-mix. I used to battle these engines all the time, now I don't have any problems with them whatsoever. I can run it for over a hour and stop and remove the seat cover and touch my engine anywhere and it is cool. If your problem is what I think it is, then do these things and you will be fine.

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    • #3
      Re: 95 SLX 780 Engine Problem

      Oh, and for the gas in the cylinders, that could be a by product of your 60 psi problem. Check your carbs and make sure they are not spitting out too much gas. 2 turns low side 3 turns high side. Make sure the valve is opening and letting the pressure out of the tank. When you store your ski's after a days fun, make sure the gas valve is off if you leave the tipped up, cause the gas could drain into your engine.

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      • #4
        Re: 95 SLX 780 Engine Problem

        I was told that the SL 750 had the hole in the piston problem because they have the intrigal pump. These already have the remote pump attached to the hull. The piston that burned up was the #3 and not the #1. Are you sure we are talking about the same ski. If the cylinder head is nice and smooth can I just replace the pistons and rings?

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        • #5
          Re: 95 SLX 780 Engine Problem

          you can if the cylinder is smooth and as long as you know the size of piston and rings you need. SBT may have the size. I know they rebore. As for your 750, these engines are famous for piston issues. I gave you 1 example of a cause. But there are more. I have experienced where these engines run too hot and that makes for early firing of the piston and causes the hole or other breakdowns due to heat expansion within the cylinder. Causes of overheating? Circulation issues due to defective thermostat. When the thermostat is working fine, I feel it still restricts the flow of water too much. I remove all my thermostats on my Polaris Jet Ski's. My feeling is you need these engines to run cool, I am also referring to the 93-97 engines. Like I said earlier, once I made the changes, my problems went away. Changing engines is a bitch on those polaris's, after you do it a few times, you figure out why it is happening and how to resolve it. This worked for me. I have seen a lot of these year jet ski's on ebay with blown pistons for sale.

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          • #6
            Re: 95 SLX 780 Engine Problem

            What do I do with the wires that go into the electrical box once I remove the thermostat off?

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            • #7
              Re: 95 SLX 780 Engine Problem

              those wires are to a sensor that just keeps track of the water temp for overheating. But all it does is maybe sometimes make your light flash on your ski. Depending on what ski you have. Some had a buzzer. Don't worry about the wires. Just undo the 2 bolts and remove the thermostat and spring and put it back together. Use some silicone to make sure you dont get a leak. There is not much pressure anyway, just flowing water from the pump forcing it.

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              • #8
                Re: 95 SLX 780 Engine Problem

                That's easy enough. I really appreciate your advice. One more thing though, Since this already has a remote fuel pump I should just then convert this to a premix. Once I install the kit should I remove the oil tank and all hoses? Also, what ratio should I use and how do I measure. Not sure I understand this. Thanks!

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                • #9
                  Re: 95 SLX 780 Engine Problem

                  Just plug the hoses that go to the oil tank. The small lines to the carb from the oil injection, you will have to plug also. There is a block off kit you can buy from SBT. Do remove the oil injection from the front of the engine, cause it will burn up with no oil running through it. Mix 13 ounces of oil to 5 gallons of gas. Thats the easiest way to tell someone. Syphon your gas out into a 5 gallon can, thats easy enough. Then mix the oil into a true 5 gallons of gas and fill up your tank. Fast.

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                  • #10
                    Re: 95 SLX 780 Engine Problem

                    Ok. I really appreciate your advice and I will get on all this as soon as I get all y parts. Thank you again for you advice. You were very,very helpful.

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                    • #11
                      Re: 95 SLX 780 Engine Problem

                      no problem

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                      • #12
                        Re: 95 SLX 780 Engine Problem

                        I have another question. I pulled the # three piston and it was pitted along with the head. The rings were intact (Not broken) so I checked the reeds and they were also good. What else could brake in the engine that could possibly get into the chamber.

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                        • #13
                          Re: 95 SLX 780 Engine Problem

                          well whatever metal has broken off the top of the pistons or the cylinders or whereever has to go down. Gravity. If I were you, if you have not already taken the engine out and laid it on a bench, I would. The removed all the cylinders and then removed the lower end and work from the crankshaft up. Clean out the lower engine and make sure the seals are all good. You can tell that by a dry Stator. If there is oil in the stator area, the your front seal is leaking. Best to do this right. Sometimes if you just do enough to get it running, another problem will pop up and you spend 3 times it takes, instead of just doing it through in the first place. When I rebuild an engine, I break it all the way down and rebuild it completely. Then I know it is a solid engine. Fast

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