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  1. #1
    Piston? WHAT Piston?!? JGringo's Avatar
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    Pros, I need Help with overheating sl780 issues...

    Hey all,
    I have a polaris 780 in a 750 hull with an extended pump (I believe to be a 144). Its pretty much a swapbox.

    Engine specs:
    -Lightweight RAD Flywheel
    -Stock Carbs w/ Tau Ceti's and Wraps
    -Stock reeds/cages
    -External Fuel Pump
    -Lightly polished exhaust port areas
    -Stock Shaved Heads to 130?? psi
    -650 dry pipe exhaust
    -Aftermarket waterbox of some sort

    The front (mag?) cylinder overheats. I tore the entire engine down to individual parts and cleaned/scrubbed/blew out the waterjackets and now every passage is clear and it still overheats real bad. Just test running the machine, the back two cylinders are not nearly as hot as the front one.

    I was wondering if the cooling line that runs from the extended pump might not be meant to cool the 780 engine and is really intended for cooling a 750 engine? I am debating putting a secondary 3/8'' line off the pump and running it to the exhaust waterjacket of the front (mag?) cylinder by drilling and tapping a new inlet on the exhaust manifold. Would this be a bad idea? I have had this ski for 2 years and have had this problem the whole time.

    Note: I just recently switched to the dry pipe, I had these issues with the stock 780 wetpipe exhaust.


    Also, Anyone know where I can get the middle top bolt for the exhaust manifold of the sl780 engine? It is the longest screw and I can't find it at any hardware store.

    My ski:

    I have circled where I would put an inlet for a secondary cooling line.




    Please Help,
    -Gringo


  2. #2
    bowsniper's Avatar
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    Welcome to the HULK! Intake water hose from pump area connected ? Properly routed?? Mine wasnt on my slt and it almost smoked the center and the pto.

  3. #3
    Moderator beerdart's Avatar
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    Are you testing in the water or on the hose?

  4. #4
    Piston? WHAT Piston?!? JGringo's Avatar
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    Thanks, I have been on and off of here for awhile...

    I forgot to mention that I replaced the thermostat unit in the cooling bar that goes on top of the cylinder heads. It is not wired.

    The guy I bought the ski from clipped all these wires that go to sensors trying to fix other problems with the swapbox and now it looks like frankenstien.

    What is not connected:
    -Fuel sensor
    -Trim motor
    -Thermostat
    -Other wires that I don't know what they go to.

    It has spark in the correct timing and is not fuel injected.

    I assume the things that are not connected would not effect how it runs as long as there is compression, heat, spark, fuel and combustion. It does run horribly but that might be due to needing a full gasket rebuild.

  5. #5
    Piston? WHAT Piston?!? JGringo's Avatar
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    Tested in the water.

    It has no bottom end what-so-ever, and I had it professionally jetted and tested in a tank. Once it planes out, it goes like a bat out of hell, but in order for me to get it there I have to do what is considered a "no-no" by getting the pump out of the water, jacking the revs up high and dropping it back in. This was done with the trim at top, center, and bottom locations.

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    First of all, have you verified that the head gaskets are oriented the correct way? They go on a specific direction, or you will overheat.

    The 3/4" pump supply hose is totally fine for a 780. It uses the exact same thing for a 780 as a 750, as well as a 1050 or 1200.

    There should be a "T" from the 3/4" hose going into the exh manifold.

    You then will need to run the 3/4" hose into the exh pipe.

    There's a brass fitting at the top of the dry pipe that needs to flow overboard. Either through a pisser fitting or "T" into your cooling water outlet. This prevents air pockets from getting trapped inside the cooling system.

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by JGringo View Post
    Thanks, I have been on and off of here for awhile...

    I forgot to mention that I replaced the thermostat unit in the cooling bar that goes on top of the cylinder heads. It is not wired.

    The guy I bought the ski from clipped all these wires that go to sensors trying to fix other problems with the swapbox and now it looks like frankenstien.

    What is not connected:
    -Fuel sensor
    -Trim motor
    -Thermostat
    -Other wires that I don't know what they go to.

    It has spark in the correct timing and is not fuel injected.

    I assume the things that are not connected would not effect how it runs as long as there is compression, heat, spark, fuel and combustion. It does run horribly but that might be due to needing a full gasket rebuild.
    Have you installed the thermostat up side down? Have you verified the thermostat opens at 143*?

    Quote Originally Posted by JGringo View Post
    Tested in the water.

    It has no bottom end what-so-ever, and I had it professionally jetted and tested in a tank. Once it planes out, it goes like a bat out of hell, but in order for me to get it there I have to do what is considered a "no-no" by getting the pump out of the water, jacking the revs up high and dropping it back in. This was done with the trim at top, center, and bottom locations.
    Why do you suspect you have a 144mm pump? Those were a custom build. It's possible the impeller is too large for a normal 780. (92+ HP)

  8. #8
    Piston? WHAT Piston?!? JGringo's Avatar
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    I didn't use head gaskets. I used a copper head-gasket sealant. I didn't feel like paying for new head gaskets because I assumed (I know, assuming makes an a$$ out of you and me) that using the sealant would not effect the cooling.

    Do you have a pic of the way that the thermostat is supposed to go it? I thought that I installed it correctly, but you never know.
    **Remember, it is not wired up however it is spring loaded so I assumed if it is not wired up, it shouldn't effect it.**

    As for the pump, I don't really know. The ski is in Milwaukee and I am in Houghton, MI in the Upper Peninsula for School at Michigan Tech University. I bought an impeller off of a guy on here and didn't know the pitch, all I know is that it matched identically what was in the pump originally...

    If I know the i.d. on the impeller, would you know what pitch it would be?

  9. #9
    Piston? WHAT Piston?!? JGringo's Avatar
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    Looking back at old PM's I sent out in search of a new prop, I found my pump is a 148mm pump. As for the pitch of the impeller, it might be a 16-21 but I do not know.

  10. #10
    seaobin's Avatar
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    [QUOTE=JGringo;1377330]I didn't use head gaskets. I used a copper head-gasket sealant. I didn't feel like paying for new head gaskets because I assumed (I know, assuming makes an a$$ out of you and me) that using the sealant would not effect the cooling.

    Ding Ding Ding, The head gaskets also regulate how much water goes through he cylinder jacket.

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