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  1. #1
    Yamaha artisan Cutlass's Avatar
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    Fuel Tank pressure...help me understand

    So I've done some reading about the subject, but I'm hoping to understand it better. I'm not having any problems, I just want to understand the subject a bit better and my GP1200 and Mikuni don't really describe this in any way. After riding the ski around for a few minutes, I should have pressure in the tank and should feel it if I pop the gas cap off, correct?
    So WHAT creates pressure in the tank when the engine it running?
    Also the fuel tank vent is there to let air into the tank as the fuel level drops and pop off pressure if the tank pressure is too high, correct? I'm guessing its not just an open vent to prevent fumes from building up in the engine compartment, right? Or does tank pressure also help to keep the fuel lines packed with fuel for easy startups?


  2. #2
    GoFastGuru's Avatar
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    pm me

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    TJBrad04's Avatar
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    replace the vent valves and check valves, i see them go bad a lot in seadoos too

  4. #4
    GoFastGuru's Avatar
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    builds pressure in tank out return line because the carb has a small return line hole to keep back pressure in carb. it is pushing fuel back in tank.and increasing pressure on inlet side

  5. #5
    PolarisNut's Avatar
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    Quote Originally Posted by GoFastGuru View Post
    builds pressure in tank out return line because the carb has a small return line hole to keep back pressure in carb. it is pushing fuel back in tank.and increasing pressure on inlet side
    With all due respect, that is not how it happens. It defies the laws of physics. You cannot build pressure in a closed tank by pumping less liquid in than you pulled out. Say the fuel pumps are pulling 10oz/min and the engine is burning 7, that means 3 goes back to the tank. You have a net loss of 7oz of fuel, so there is no way this would contribute to pressurizing the tank. The pressure actually comes from the fuels own vapor pressure. It's the same reason why a red fuel can swells up when left in the sun. Some of the fuel vaporizes, and pressurizes the can/tank.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    So I'm thinking that running the engine exposes the fuel to some heat (as it circulates through the warm carbs) which speeds up the vaporization/evaporation and helps create pressure. That and it being stirred up from circulating back into the tank.
    I found another great thread on the subject. Very informative. http://www.greenhulk.net/forums/show...-tank-pressure
    Seems like the subject is very well discussed and understood around here...until the last post in that thread where the ISLANDRACING guy stays they set skis up with or without fuel tank pressure and it doesn't seem to matter. And the jetboats with the very same 65U and 66V engines have open vented gas tanks with no issues. The subject is as clear as mud.

  7. #7
    Quote Originally Posted by PolarisNut View Post
    With all due respect, that is not how it happens. It defies the laws of physics. You cannot build pressure in a closed tank by pumping less liquid in than you pulled out. Say the fuel pumps are pulling 10oz/min and the engine is burning 7, that means 3 goes back to the tank. You have a net loss of 7oz of fuel, so there is no way this would contribute to pressurizing the tank. The pressure actually comes from the fuels own vapor pressure. It's the same reason why a red fuel can swells up when left in the sun. Some of the fuel vaporizes, and pressurizes the can/tank.
    come on polarisnut, IF that ski is moving [ which its intended to do ] fuel is being sloshed all over the place, the last time i put fuel in a closed container and shook the shit out of it, it developed alot of pressure, and would stay till the cap was released, its intended is to maintain pressure at all times, even when cool.

  8. #8
    Moderator beerdart's Avatar
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    Yamaha is one of the only manufactures that only uses a vent in valve with no out vent. Also remember the pressure in a system is equal so the return is seeing the same pressure build up as the supply. Do you need to rejet for summer gas vs winter gas as the reed vapor pressure is lower in the summer? I think not. This discussion is Silly I tell ya Silly...

  9. #9
    PolarisNut's Avatar
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    Quote Originally Posted by 123456 View Post
    come on polarisnut, IF that ski is moving [ which its intended to do ] fuel is being sloshed all over the place, the last time i put fuel in a closed container and shook the shit out of it, it developed alot of pressure, and would stay till the cap was released, its intended is to maintain pressure at all times, even when cool.
    It is still caused by the fuel evaporating, and slightly pressurizing the can. When you shake the fuel, the light ends evaporate more quickly, accelerating the pressurization. Not sure what your point was there.

    EDIT: I'm thinking about installing a small compressed gas canister filled with nitrogen (think paintball gun) and hooking it up to the fuel tank, so I can pressurize it immediately after fill up. I bet the engine would start without even touching the starter button. If I can do 80+ without fuel tank pressure, imagine what I can do with a 2000psi tank of nitrogen I could also make a pressure vessel fuel tank and set new records.

  10. #10
    GoFastGuru's Avatar
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    the return line is at top of tank not having a pickup setting in fuel. the fuel line goes to the bottom of tank in fuel. disconect both lines and blow in return line an you will get fuel flowing out in line blow in fuel line you will not get fuel put a air compresser on check valve it will relese pressure at 6 to 8 pounds not trying to start any trouble

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