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  1. #1

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    Fuel tank pressure question

    Ok, I have two SUV that I rebuild, I rebuild the first one last year and it is giving me problem. It will bog at low speed when I give it gas. I have to give hit wot and it will run at top speed. I was rebuild the second SUV so I figure I will address this issue later. After sitting for a month, I bring it out for testing, to my surprise the problem is gone, I have a whole day of fun without any issue, but the at the end of the day the problem start again. I winterize it and continue on the 2nd SUV. This year I fishing the 2nd SUV and took both out for a test run. To my surprise again the 1st SUV is running fine again the whole day. But then the problem came back again. The 2nd SUV just running fine.

    I rebuild both exactly the same and both are stock, I figure it is not the carbs as they both rebuild the same way and past all the test. I did notice one main different thought. Both SUV were sitting outside under the sun with half tank of fuel. When I took out the fuel cap of the 2nd SUV, I can hear the air comming out of the tank, but when I took out the fuel cap on the 1st SUV with problem, no air come out. I try it again for two more days, same result. That bring me to believe that the problem with the 1st SUV is that the fuel tank didn't hold pressure.

    Now my question is, how to do test the tank pressure? How much pressure should it hold? Where can I find a step by step testing procedure? Thanks.


  2. #2
    There is a spec but I don't have it at hand... perhaps 5psi? There are two little valves just above the tank, one lets in air so a vacuum cannot be developed in the tank (making you run LEAN), and the other allows some PSI to build. I think the reason why the pressure is allowed to build is to prevent water from migrating it's way into the tank through a bad gas cap seal because of all the water flowing around it during riding. That would be a very bad combination. It might help a little get fuel into the carbs, but the carbs should NOT depend on that, I don't believe. Their pressure is regulated to ambient air, so if the pump is able to produce full flow (good diaphrams, good one-way valves, etc) and there are no restrictions (bad/clogged filter), then the ski should be able to run just fine with NO pressure in the tank.

  3. #3
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    test the valves alone, the fuel tank is obviously a high volume bit and it takes a long time to pressurize it, unless you are checking a fuel system problem

    3-5 psi on the valves..make sure they are installed correctly., Tank pressure helps fuel flow and warmed fuel is always getting back to the tank, so you should get a short his when opening the cap * assuming the cap gasket is good)

    ANY brand of tank valve is good. Some vendors charge $20..others $5

    Tank pressure is a good reason to turn your fuel tap off when done for the day. Too much tank pressure can force gas past the carbs and into the engine, with unhappy results.

  4. #4
    I adapted a $15 mr gasket fuel pressure gauge onto my ski. Runs 6psi at wot.

  5. #5

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    Quote Originally Posted by ZeroClient View Post
    I adapted a $15 mr gasket fuel pressure gauge onto my ski. Runs 6psi at wot.
    Cool, do you have pictures, I would love to add that to mine.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    I'm also a believer that tank pressure (or lack of) could NOT affect how the engine runs. Tank vacuum could affect it though. For example, if the tank isn't venting properly and a vacuum builds in the tank. That vacuum will fight against the carb fuel pumps.

  7. #7

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    Cutlass, I think I'm going to have to disagree with you on this one. I think that the pumps in the carbs are set up to pump fuel, not to suck fuel from the tank. It is in everyones best interest to have a few psig on their tank. This pressure pushes the fuel to the carbs so that the diaphrams in the carbs can do their jobs.

    To much tank pressure, however can cause fuel to leak directly from the carbs to the crankcase. The high pop off pressures that we originally have in the stock set up was put there (in part) to keep this very thing (leaking) from happening.... this and the fuel shut off switch that NMpeter was refering to.

    It is all a balanceing act though... I agree with you there: to little pressure, you have problems... to much pressure, a different set of problems. I keep hearing different pressure ranges from different members. Mine ran years ago at 3.5psig.
    Last edited by salty; 08-29-2013 at 01:22 AM.

  8. #8
    mudslanger's Avatar
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    Quote Originally Posted by salty View Post
    Cutlass, I think I'm going to have to disagree with you on this one. I think that the pumps in the carbs are set up to pump fuel, not to suck fuel from the tank. It is in everyones best interest to have a few psig on their tank. This pressure pushes the fuel to the carbs so that the diaphrams in the carbs can do their jobs.

    To much tank pressure, however can cause fuel to leak directly from the carbs to the crankcase. The high pop off pressures that we originally have in the stock set up was put there (in part) to keep this very thing (leaking) from happening.... this and the fuel shut off switch that NMpeter was refering to.

    It is all a balanceing act though... I agree with you there: to little pressure, you have problems... to much pressure, a different set of problems. I keep hearing different pressure ranges from different members. Mine ran years ago at 3.5psig.
    +1 Tank pressure is a must. Besides it has already been discussed.

  9. #9

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    ok, I finally did a suck and blow....... well, i pulled out the tank valve and suck it and the air comes in fine and then I blow, and the air goes out fine. So the valve is bad the tank will not build up pressure. I will replace the valve and give it a run........I will keep posting. By the way I really like to add a fuel pressure gauge to it since it is less then $20. would someone show me how?

  10. #10
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by birdgod View Post
    ok, I finally did a suck and blow....... well, i pulled out the tank valve and suck it and the air comes in fine and then I blow, and the air goes out fine. So the valve is bad the tank will not build up pressure. I will replace the valve and give it a run........I will keep posting. By the way I really like to add a fuel pressure gauge to it since it is less then $20. would someone show me how?
    I had a fuel pressure gauge on mine for a while so I can help you. You have to do the Carburetor Drilled Return Pressure Balancing Modification in OsideBill's how to sticky. Click on the link for more info. But instead of using an inline barbed hose union...you use a barbed hose tee. Tap threads in one end of the tee, screw in the appropriate carb main jet as a restrictor, and hook up a fuel pressure gauge to the 3rd barb.
    If you don't want a permanent gauge, you just pull the tee out and replace it with a barbed hose union. Just swap over the restrictor jet.

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