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  1. #1

    SL750 Constant Backfires, Wont Run...

    My ski was running great after the rebuild, the fixed fuel system, and then the pop off increase. This was 2 days ago. Yesterday, i had it all ready to take out for the first time and went to start it, and it ran horrible. Rough start, bogging, sounded like it was choking on gas and running on one cyl. I assumed the pop off was too low due to it doing this exact thing the day before, only i could see gas on the top of the butterflys at idle, and i couldnt now.

    I pulled the carb today and checked the pop off, and it was about 1ish psi lower than i left it, but i put it up to about 18 anyways. Put the carb back in and it still barely starts, and backfires constantly.

    The only thing i can think of that changed since two days ago when it started and ran fine was i drained the tank and put 12oz of polaris oil in for a 50:1 premix with the almost 5 gallons of 87 octane in the tank, i have a 5 gal jug of 93 50:1 premix i was going to mix it with... But nothing else changed and now it dosent run, what happened? And why is it doing this?


    I did not kill it, i let it idle and it backfired and stopped... The PTO is constantly backfireing, and the MAG and CEN backfire ocasionally. I dont see gas pouring in anywhere, is this too much gas or something? I really think i need a whole carb from a running 95 750...


    This is 2 days ago...


  2. #2
    ph2ocraft's Avatar
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    What's your compression and how do the plugs look?
    Do you see any fuel come out of the carbs when it back fires?
    Possible reeds?
    Be sure crank index is O.K. (make sure there is no water in the cylinders).
    Is it possible the stator moved? Did you remove it during the rebuild and posibly forget to tighten it?

  3. #3
    Quote Originally Posted by ph2ocraft View Post
    What's your compression and how do the plugs look?
    Do you see any fuel come out of the carbs when it back fires?
    Possible reeds?
    Be sure crank index is O.K. (make sure there is no water in the cylinders).
    Is it possible the stator moved? Did you remove it during the rebuild and posibly forget to tighten it?
    Im scared to do a compression test, but i think i will shortly and probably put new plugs in...

    No visible fuel at any point with the high pop off.

    My reeds are in poor condition and about 1/8" off the lip, most of them are chipped, but it ran fine 2 days ago, and not 12-14 hours later...

    I suppose i will pull the heads and see whats up.

    Stator is tight as hell, i didnt want it moving, but it is possible... Would there be a way to check timing roughly, or easily?

  4. #4
    ph2ocraft's Avatar
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    Quote Originally Posted by ZOMGVTEK View Post
    Im scared to do a compression test, but i think i will shortly and probably put new plugs in...
    Compression first.

    No visible fuel at any point with the high pop off.
    I mean fuel popping out of the carb throats when it backfires.

    My reeds are in poor condition and about 1/8" off the lip, most of them are chipped, but it ran fine 2 days ago, and not 12-14 hours later...
    If they were chipped and flapping at like 9 zillion MPH, they may have finished cracking or breaking, You really need to inspect AND replace the reeds.

    I suppose i will pull the heads and see whats up.
    Compression and reeds first.

    Stator is tight as hell, i didnt want it moving, but it is possible... Would there be a way to check timing roughly, or easily?
    You have a timing light?

  5. #5
    No timing light... Compression is 120P/120C/125M.

    Heres some pics, might explain why the CEN and MAG were running hot on the hose...

    There is about 2-3 mins on these pistons.



    PTO




    CEN




    MAG




    Looks like the CEN and MAG are a little low on oil there, the walls are almost dry.
    Last edited by ZOMGVTEK; 08-01-2008 at 11:42 PM.

  6. #6
    Everyones talking about how super fast their running skis are but not telling me why my ski is broke.

    Sooo uhhh... Why is it broke?

  7. #7
    It ran fine for about 30 sec off a cold start, even started good...

    Then the starts to follow were hard, and it was bogging.

    Its kinda like the timing is off, but how would that happen? Stator got knocked out of alignment or something?

    Could it be reeds?

  8. #8
    It appears the analysis is, the thing starts OK, but it needs some gas to get going, after about 2 seconds it idles fine and it will continue to idle. As soon as you start revving it, it boggs and backfires and usually kills itself. Cause of this?

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Sounds like 1 of 2 things.

    Heat related: ohm test the stator and check to see if it's within specs

    Fuel related: Did you do ALL the fuel system updates? Hoses, petcock, filters, and carbs.

    If your reeds were borderline, I'd be looking at those again. Does fuel spit out of the carb top when it does run?

    Have you done a leak down test?

    Is the auto cock removed with all hoses capped air tight?

  10. #10
    Quote Originally Posted by xlint89 View Post
    Sounds like 1 of 2 things.

    Heat related: ohm test the stator and check to see if it's within specs

    Fuel related: Did you do ALL the fuel system updates? Hoses, petcock, filters, and carbs.

    If your reeds were borderline, I'd be looking at those again. Does fuel spit out of the carb top when it does run?

    Have you done a leak down test?

    Is the auto cock removed with all hoses capped air tight?
    I will check the stator later today.

    Currently i have a line running from the reserve on the tank dirrectly to the tripple outlet pump, nothing in line. Carbs are in good shape, rebuilt with about 4 hours on them, clean internal filters. I have fuel pressure on the return.

    No fuel is spitting out of the carbs, just some fire when you rev it.

    No leak down test done.

    Never had a autocock.

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