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  1. #1
    Piston? WHAT Piston?!? JGringo's Avatar
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    780 Front Cylinder is Red Hot, please help!!

    Hey everyone,
    I have a slightly modified 780 engine in a 750 hull. The engine has stock carbs (re-jetted), RAD Flywheel, Short-dry pipe, Tri-oval Waterbox, lightly ported exhaust (sanded all around), and Tau Ceti FA's on the carbs.

    The problem I am having is that the front cylinder is really hot and the engine just doesn't have the power that I want it to. It just doesn't have any kick. I took it in to a shop and they told me my compression was off too much by 5 psi in the rear cylinder. He also told me that I don't have a thermostat in the engine which causes the front cylinder to heat up really hot.

    I fixed the compression already and have yet to get a thermostat. I don't know what the thermostat would do to help this heat problem. The cooling lines are all routed correctly and the engine is sealed up nice and tight.

    When I bought the ski, so many sensor wires were clipped and the trim motor wires were even clipped. If I install the thermostat, I won't even be able to read if the engine is hot or not. Do you guys have any suggestions?

    P.S. the ski was raced at one point, but the whole engine is was not (Except for the CDI box-which is why the wires are all clipped).

    I thought about rigging up a direct cooling line to the front cylinder to stop this heat problem, but that seems a little unnecessary.

    Please help,
    -Jeff


  2. #2
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Are you checking the temp on the hose through the flush fitting? Or on the water? It's normal for the mag cyl to be a good bit hotter than the others when on the hose. Also without the t'stat and on the hose the path of least resistance is allowing most of the water out the water outlet hose. If the t'stat was in place the water would be forced through the engine. When flushing, you will need to pinch off the water outlet hose w/o the t'stat.

  3. #3
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    head gasket

    don't the fuji motors require that you index the head gasket in just a certain way to correctly flow water around the cylinder and up into the dome??

  4. #4
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Yes they are to be positioned properly, but the openings are large enough that its not as critical as the 650-750s. They had alot smaller openings
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  5. #5
    Piston? WHAT Piston?!? JGringo's Avatar
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    okay... what if I am not running the head gaskets? I just have the copper head gasket material. Also, it was running hot on the water, so hot it burned the back of my hand. I have NO thermostat at all. I was told this would fix my issue but I do not know. I am trying to get one this weekend and test it on the water.

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    The thermostat will not help the MAG get cooler. I don't even know how that could be possible?

    Only the MAG cyl is hot? Maybe something is blocking the cooling openings?

    Did you use a RTV sealer on the head gaskets?

    Check to see if the cooling bar is plugged up on that cyl.

    Is the stator cover hot to touch from the front side of the cover?

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by xlint89 View Post
    The thermostat will not help the MAG get cooler.

    I don't even know how that could be possible?...
    The thermostat does affect (improve) MAG cylinder cooling when running on the reverse flush garden hose connection, but not when running in the water.

  8. #8
    Piston? WHAT Piston?!? JGringo's Avatar
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    Nothing is blocking the cooling lines, the flywheel cover is not hot, and the thermostat location at the exit part of the bar has absolutely nothing in it to cause any resistance in the cooling lines. I was told that this resistance would cause/force the water to flush through the front cylinder more. The cylinder itself is runnable as long as I don't just go full throttle the whole time...

    I was looking at www.worldofpowersports.com to find a phish diagram of the cooling lines and thermostat parts. It looks like there is supposed to be some sort of spring system and resisting blockade within the end/exit location of the bar. Does anyone have a picture of what is supposed to be in the inside of the "exit" part of the bar?

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    You can also find the online parts diagrams from Polaris here.

    But I don't think the non-present thermostat is the problem.

    Have you reverse flushed the water intake hose connection, from the engine end of the hose right out the jet pump water intake screen?

    Make sure that intake screen is in place inside the jet pump nozzle, and is not plugged up.

    And make sure that little plastic screen is facing the right way. The little holes are supposed to be angled forward, to catch the water flow.

    And you have confirmed that the cooling water flow is indeed correctly hooked up, all the way from the jet pump supply hose through to the water exit out the hull?
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  10. #10
    Piston? WHAT Piston?!? JGringo's Avatar
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    That would definitely cause some resistance within the cooling lines and force more water to run through the front cylinders, don't you think? My bar is just like the picture on the right, empty...


    Wouldn't all the cylinders get hot if the cooling lines were blocked?


    I FORGOT to say that I am running a 650 dry pipe with this engine. Would that have anything to do with it?

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