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  1. #1
    gski's Avatar
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    Differential pop off pressures triple Super bn

    Three carb super BN.
    I want to know if a differential of 5 psi higher pop off in one carb could have caused a lean mixture to burn up the piston.
    (I think this is the ahh hhha moment for the root cause.) I tested all three carbs with a home made tester and two started to leak or pop off at approximately at 10 psi and the third at 15-16 psi. (The third was the burned up one.) So i removed and put the old springs back in the first two and now all three start leaking at 15-16psi. So now the question ... am i going to burn up all three pistons now with the higher pop offs or did i fix the lean carb to open at the same air fuel mix as the other two and all will be ok or do i need to lower all three to the 10-12 psi range?

    needle/seat is 2.0 , factory jets. On a stock 96 SLX 780 polaris.

    And yes i rebuilt all three carbs and now a new cylinder and piston.
    i have seen the charts, but i dont know which pop offs or pressures im suppost to have.
    Thanks


  2. #2
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    I doubt the difference in popoff burned down the engine. Spec is actually 20-24 PSI, using the black 80g spring and 2.0 needle and seat. As long as you are using the proper spring and needle/seat combo, you generally don't have to worry about popoff on these.

    I would check your whole fuel system. Which cylinder burned up? You say the third, but is that from the front or the rear? You may have an air leak somewhere, and I would look there after going through the fuel system.

  3. #3
    gski's Avatar
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    Thanks for the reply. I took apart the intake down to the case, cked the reeds,seals and intake. All looked good. The pto Aft piston burned up at the exhaust side. Dont know how to check the seal at the crank, but i would think that an air leak there would lean out the whole crank case and cause the other pistons to burn as well. I just think that during acccleration and deceleration above and below the idle and high speed jet circuits, that if the one carb was running lean from a higher pop off pressure that it would cause this. Just dont know if 5 psi would do it. However most of the time the ski is being run in that pop off circuit range. So i dont know...
    Since the charts say the lowest psi for the 2.0 should be 16 psi, i went with that. Just dont know why i would have gotten two 10 psi readings on any spring that was in the rebuild kits because none of them should be that low.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Each crank case section is effectively sealed from the others. Each cylinder essentially runs as a single cylinder engine.

    The rear crank shaft seal can leak air and only affect the PTO cylinder air:fuel ratio.

    Are you testing pop-off with the carburetors completely 'dry'?

  5. #5
    gski's Avatar
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    not completely dry. just a little wd40 and listening for the pressure to start leaking in. I know thats not how you are suppost to do it. But the modified foot pump and guage is not the ideal test equipment to do the test. All three carbs now hold pressure at approx 10 psi and with half throw of the foot pump it starts to open the needle at approx 16 psi and leaks down. before the first two were leaking down at 10 psi.

    Btw: i replace the thermstat it opened at 180 and then subsquently at 160's. so i replaced with the oem 143. Thinking that the pto cyclinder has the smallest cooling hole in the manifold.
    And all three spark plugs were black showing rich condition.
    And all jet setting were set just slightly rich from factory settings.
    And two check valves work. Cleaned fuel tank. replace all fuel lines, rebuilt fuel pump. Added large inline fuel filter. Rebuilt carbs with possiblly the wrong (weak) springs in the first two.

    I really dont want to do the leak down test with the racket balls if i have to take apart the exhaust. Since the carbs are still off, is there an easy way of doing the test?

  6. #6
    BlueFishCrisis's Avatar
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    Quote Originally Posted by gski View Post
    Added large inline fuel filter.
    OEM? If not, this may cause additional fuel delivery issues. If you have the OEM fuel / water separator, the mesh screen that is stock in the assembly is sufficient for fuel filtering.


    Quote Originally Posted by gski View Post
    I really dont want to do the leak down test with the racket balls if i have to take apart the exhaust. Since the carbs are still off, is there an easy way of doing the test?
    No easy way. Must fully seal off the intakes and the exhaust outlet. The easiest way you may be able to accomplish this is to create a block off plate to install in place of the exhaust manifold-to-exhaust pipe gasket to effectively block the exhaust, then bolt the pipe back in place, block off the intakes, then pressure test via the pulse fitting.

    I didn't want to take my exhaust apart the first time either, but my desire to prevent a burn down convinced me otherwise.....

  7. #7
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    Make some block off plates to do the leak test. Use gaskets as templates. I made mine from 3/8 aluminum and made gaskets out of rubber. Pressurize the engine with 10 PSI and it should hold pressure. Im not happy until I see its perfectly sealed. Its just peace of mind knowing an air leak isnt going to kill your engine.

  8. #8
    gski's Avatar
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    Thanks for the replies guys. Ok you convinced me to do the leak check. I made the intake block off plates today. Now i got to figure out how and where to make the exhaust block offs. May take another day to get material.
    I'll take out the additional inline filter as well. My other slt780 had one with no problems, so i put one in. Ya, the carb filters should be adaquate with the oem water sep/filter.

  9. #9
    gski's Avatar
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    Well think i found another "ahhhhaa" problem. Made the block off plate for the exhaust/expansion. Bought gasket material from Napa. Installed, then pressurized to approx 10 psi and stood back,and watched the pressure drop slowly. Almost to the point where your second guessing yourself and the block off points. Pressurized it again to approx 16 psi. Then i sprayed HEAVILY the whole engine in windex, looking for bubbles. Kind of a last ditch effort. And then there it was. Bubbles from the aft crankshaft seal.
    So, looking at the Polaris for dummy's repair manual, it looks like remove engine and break the case halves open to change the seals. I'm kind of distraught. Or mistaken?
    Need parts... again...
    Thanks guys

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by gski View Post
    ... i sprayed HEAVILY the whole engine in windex, looking for bubbles...
    Tip: Use a pump spray bottle with regular dish washing soap (hand washing dish soap) and water. Makes much better bubbles

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