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  • SL750 Pop-off Spring

    I have a few questions regarding the pop-off pressure on a 93 SL750. I have just installed a new SBT motor and rebuilt the carbs and am having issues with the carb set-up. The pop-off is too weak and actually flooding the engine while starting.

    1 - My first question is if you supply Mikuni parts in your rebuild kits? I ask because I tried the lightest spring in the kit in an attempt to increase the pop-off and the pressure was double what I had with the original spring.

    2 - This leads to my second question, did Polaris set these carbs up with springs that are not standard? I noticed in Polaris parts diagram they list the spring as Mikuni # 730-03032 but this is not one of the 4 springs listed in Mikuni's manual.

    3 - What do recommend as a starting point. The original spring was popping around 17psi (Polaris specs 10-18psi) and the smallest spring in the rebuild was holding beyond 30psi (my gauge only reads up to 30). Thought I may have installed the new spring incorrectly, but repeated the test with the original and had the same result. I know I need to increase the pressure since it is leaking but 30psi seams a bit much. I am considering ordering the spring from Polaris to see what it turns out to be, but wanted to see what anyone knew about this first.

  • #2
    Re: SL750 Pop-off Spring

    We include the two most popular springs in our kit, as do the Mikuni kits. Yes they are all standard tension, as are your stock springs. The other two sizes are dealer-only. There is no need to change your springs if you are not changing your equipment such as your air box. You should not be using the new springs, they are the wrong tension for your setup. If you are leaking at those pressure, something else is wrong. Make sure you have the arm bent to the proper position and the diaphram is not lifting it when you install the cover.
    SBT Tech Support is here to help with your problems.
    We try to answer each question quickly and accurately.
    Please do not use Private Messaging for Tech Support, use the forums.

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    • #3
      Re: SL750 Pop-off Spring

      The springs that were in the machine are physically smaller than any it the rebuild kit, and I actually got all four standards sizes in the kit. I forgot tom mention that I have made some changes to the fuel system by upgrading to a new fuel pump with 3 outputs. My concern is that this has increased the supply pressure to the Mag and Center carb slightly since they are no longer in series. Do you know what the fuel pressure is from the stock pump and the 3 output?
      Regarding the arm and diaphram, I checked pop-off before and after removing the diaphram and noticed very little change, but I agree I need to confirm the arm position. If that checks out, I'm not sure how to proceed since the standard springs in the kit make too large of a change in the pop-off. Is it not correct that the spring in the carb should have matched one of the standard springs in the kit (I didn't understand your comment "The other two sizes are dealer-only").

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      • #4
        Re: SL750 Pop-off Spring

        I wasn't aware our kits came with all four sizes now, sorry.

        If your springs were shorter, someone has cut them, or used the incorrect springs possibly out of a Keihin kit, which is not suggested at all.

        There is no concern regarding fuel pressure. The pump can not over pressurize. Excess is routed directly back to to tank. The carbs simply tap into the stream and use what they need.
        SBT Tech Support is here to help with your problems.
        We try to answer each question quickly and accurately.
        Please do not use Private Messaging for Tech Support, use the forums.

        Comment


        • #5
          Re: SL750 Pop-off Spring

          I was thinking that the new fuel pump may have a higher pressure than the original and is exceeding the old pop-off pressure of 17psi. If this new pump is supplying fuel at 22psi (for example) the current spring wouldn't hold and I end up with the leaking.

          Right now, if everything checks out ok I think I will take the lightest spring in the kit and start clipping it down until I find a suitable pressure. The problem is the smallest spring is starting above 30psi which I can't measure, and I'm not sure what pressure I will need. I see a lot of time in the shop tearing the carbs off and making adjustments (which is no fun on this machine).

          Thanks for your advice and time, I will let you know what ends up working. If you have any other ideas in the mean time I would like to hear them. Thanks again,

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          • #6
            Re: SL750 Pop-off Spring

            The fuel pump has nothing to do with pop-off pressure. The needle does not pop from fuel pressure, it pops from atmospheric pressure. Read this article for an understanding of it: http://www.sbtontheweb.com/forum/showthread.php?t=13965

            I suggest you do not cut your springs. You tune by interchanging the springs and seat sizes, not by cutting springs. There are also articles here on rebuilding and tuning carbs, I suggest you go through those as well.
            SBT Tech Support is here to help with your problems.
            We try to answer each question quickly and accurately.
            Please do not use Private Messaging for Tech Support, use the forums.

            Comment

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