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  1. #1

    780 SLT running TOO good????

    Guys,

    I just finished rebuilding a 1997 780SLT. I rebuilt all the carbs, fuel pump, fuel lines (new), fuel petcock, all new pistons, rings, cylinders bored and honed, new plugs. I rebuilt it because it melted the MAG cylinder. I know next to nothing about 2 cycle engines but have done some 4 cycles before so I have SOME rebuild knowledge. Anyway....it fires right up and runs great but I am scared of melting another piston. I know I need to check the piston wash but that takes some time to develop and I don't want to hurt it in the mean time. Are there any other indications that would be evident if it were running lean. I'm concerned that it's actually running too fast and that might indicate it's a bit lean. I had it up to about 54 MPH today (indicated on the HUD). Does that seem too fast for this ski?

    Anything else I could check?

    Thanks,

    TOdd


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    RPM is the measure to go by, not speed readings.

    If you set the carbs to factory, then it should be OK. Many guys set the carb high speed screws to 1/4 or 1/2 turn farther OUT than spec, then dial them in closer to specification once the piston wash pattern becomes clear.

    The 780 is a strong motor. A well tuned 780 SLT can run at the speeds you are reporting.

    The built in speedo is only approximate. The built in tachometer is accurate.

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    You can try plug chops, but I'm not a fan of them. (they lie)

    As K447 mentioned, normally set the highs out another 1/2 turn and adj them IN once you get a wash pattern developed.

    Take it easy on the long WOT runs until you're happy with the wash. Quick short bursts can get you a tach reading and not risk much damage by doing it.

    What kind of RPM are you getting?

    No that speed is not too much. The MFD does read higher than actual most of the time.

  4. #4
    Well....new problem. After riding for a while today, I noticed that the base gasket on the MAG cylinder (the one that melted down before the rebuild) has basically squeezed out of the front edge (MAG side). It's leaking oil down there so I assume it's leaking air too. Any idea why that gasket would squeeze out like that. I used the gasket goo that was rec. on the SBT website....maybe that contributed???

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Base gaskets don't require ANY gasket sealer.

    Did you install the exh manifold before torquing the cyl nuts down?

  6. #6
    Well...you can bet the next set will be going on dry. Hopefully I got better gaskets this time around. I think it said the ones I ordered were some kind of graphite with some stainless steel sandwiched between. I did not install the exhaust manifold first....read that AFTER I had put it all back together. Can't see how that caused the base gasket to squeeze out though. On another note....when I put it back together (2nd time around) are you supposed to retorque the head gaskets after heating it up?

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    2 piece base gaskets are unheard of. Sounds like a head gasket that are sandwiched. What brand did you get?

    Yes, ALWAYS recheck your nuts/bolts after first hour or 2 of a rebuild.

  8. #8
    Pulled it all apart last night and those base gaskets were like wet paper. Apparently the gasket goo that I had used pretty much dissolved the gasket! Anyway....I was wrong about the new gaskets. The brand is Comedic. The head gasket is the graphite/stainless sandwich, the others are just regular gaskets. I do have some high temperature graphite gasket material in the shop, thinking about making some base gaskets out of that stuff. On another note, anyone got some good piston wash pictures on a Polaris 780? Based on what I have read and seen on here, my center piston looks perfect but 1 and 3 might be a little lean. It depends on what is considered "correct". There are conflicting pictures of "good" vs. lean that I have come across. I'll try to post some pictures up tonight if I can figure out how to do that here. At least everything was clean pulling it apart this time! Only took about 1.5 hours to the get all three jugs off so it has not been too big a deal so far. Hopefully everything will go back together as easily as it came apart. Wish me luck, new gaskets will be here tomorrow and I plan to have her back in the water this weekend.

    Todd

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