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  1. #1

    Gas tank pressure!!! It matters!!

    I am only posting this to help the comuinity. I have seen the childish bickering and back and fourth on this issue when I was trying to research this problem my self. Fuel tank pressure .. does it matter or does it not?? I have seen so many arguments back and fourth on this issue ... "". I have 4 Polaris jetskis ... I have replaced rebuilt and experienced a lot of issues over the years with them .. my latest situation I had a Polaris slt 780 that would not rev up and go and after running for a while. It would water fall fuel out of a crack by the scew hole that holds the bar that keeps the oils tank in place.. Well this ski while it had that leak would not rev up and go at all.. It would start and idle all day long and sound like it would run. But once in the water it would just bog and have no power at all. So all I did was plastic weld the tank and fix the crack. Now this ski runs lie a dream. The tank now hold pressure and will make noise every time the cap is removed now. It did not before . I also have a sltx with a complete fuel system rebuild and new hoses and filters but the tank did not hold Pressure..
    This ski has symptoms of low speed bogging and at higspeed it felt like it wasn't all there .. anything below 1/4 throttle and you would have to pump the throttle "1050 has an accelerator pump". And it would go! Turns out it would leak at the top of the tank ever so slightly where it would not build pressure .. so many resources on this site said tank pressure didn't matter .. well I am posting this that for the Polaris skis you are wrong! It 100% matters in how they perform for sure !! I have since fixe the issues on both and they run like a dream. So if you have low speed bogging hard starts and no tank pressure it's worth a look.. My Experience on both skis all my other skis hold pressure this was the issue after going through the entire fuel system.. I am only posting this to help the Community because of the lack of information on this particular subject.. I hope I helped people with this post.. thanks

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  3. #2
    Polarisitis loonatik's Avatar
    Join Date
    May 2010
    Tampa, FL
    Fuel tank pressure has been in discussion, usually it's the other way around like too much pressure. It's called "vapor lock".
    There are vent valves in the fuel routing, off the top, they hold 2-4 psi. If these don't release pressure the engine wont get to top performance either.

  4. #3
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Thank you for your posting.

    My understanding is that the pressure relief check valve will maintain about 2.5 PSI positive air pressure in the fuel tank (give or take).

    The carburetors have a return fuel hose, which means the entire fuel circuit can be under mild positive pressure if the fuel tank has pressure.

    Mild pressure in the fuel return circuit might affect the carbs differently than fuel pump pressure alone. Good working fuel pump is not providing the same overall effect as good fuel pump plus air pressure from the fuel tank.
    Last edited by K447; 08-11-2017 at 07:55 AM.

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  6. #4
    cparsons0413's Avatar
    Join Date
    Jun 2019
    Hayes, Va
    Thank you for posting this. I have been racking my brain trying to figure out the low speed bogging, hard starts and no tank pressure sound when I release the fuel cap. I rebuilt 2 separate sets of carbs with the same result. I'll see if I can't track this down.

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