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  1. #21
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Anything is possible, but I highly doubt your carbs are dumping more fuel than the pump can deliver.

    If you have the Mikuni pentagon shaped fuel pump, it delivers 65L per hour. It would not be out of the question for your plugs to be slightly wet when ran on land. Fuel requirements are much less than when the engine is pulling a load. (in water)

    Generally I wouldn't be concerned with a 1/2" of vacuum loss over 10 mins. A pin hole would decrease vacuum almost immediately.

    Was the pulse hose dry? No gas or oil inside of it?

    Was the pulse fitting loose at all?

    Were you able to try a new O ring in the separator and see if the gas level came up again?

    Test the fuel tank by capping off all the fuel pick up fittings. Then you can install a vacuum pump on the return fitting of the pick up tube and apply 3-4" of vacuum and see if it holds. Any crack or gasket leak will show up really fast. I guess you could also apply 2-3 psi of pressure as well, but don't use any more than that, or you might damage the tank. Light vacuum would be a safer bet.

    Yes, cap the fuel supply hose at the tank and install a vacuum pump on the fuel pump supply hose. That should test everything in line from the tank to the pump.


  2. #22
    Pulse hose was bone dry and I forgot to check the nipple. Will look at that tonight hopefully! Have not tried a new oring yet. Had to wait till today cus I get them from my work! Will pick another one up and try that again aswell. Also will perform the tests you’ve described above thank you!!

  3. #23

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    Quote Originally Posted by xlint89 View Post
    Anything is possible, but I highly doubt your carbs are dumping more fuel than the pump can deliver.

    If you have the Mikuni pentagon shaped fuel pump, it delivers 65L per hour. It would not be out of the question for your plugs to be slightly wet when ran on land. Fuel requirements are much less than when the engine is pulling a load. (in water)

    Generally I wouldn't be concerned with a 1/2" of vacuum loss over 10 mins. A pin hole would decrease vacuum almost immediately.

    Was the pulse hose dry? No gas or oil inside of it?

    Was the pulse fitting loose at all?

    Were you able to try a new O ring in the separator and see if the gas level came up again?

    Test the fuel tank by capping off all the fuel pick up fittings. Then you can install a vacuum pump on the return fitting of the pick up tube and apply 3-4" of vacuum and see if it holds. Any crack or gasket leak will show up really fast. I guess you could also apply 2-3 psi of pressure as well, but don't use any more than that, or you might damage the tank. Light vacuum would be a safer bet.

    Yes, cap the fuel supply hose at the tank and install a vacuum pump on the fuel pump supply hose. That should test everything in line from the tank to the pump.
    I have the exact same problem with my SLT 780, the separator only fills with gas to a level just above the bottom of the steel filter. I am only able to attain about 4500- 5400 rpm, not on limp mode....There is no swoosh of air when I open the fuel cap, I have checked and double-checked all the hose connections, all the hoses are new, the two O-rings in the separator look good, I do not have a vacuum pump here at the lake but blowing and sucking through the check valves appears to be okay ... I'm going to water test on my lift to see if anything I have done so far makes a change, if it does not, I am going to bypass the fuel selector valve with a hose fitting, if that does not solve my problem then I am at a loss as to what is causing this issue until I can vacuum or pressure check the system. I suppose I could also bypass the fuel water separator temporarily to eliminate that issue as I have no access to O-rings for it. I want to double check the fuel connections on the sender, does anyone know for sure which fittings are for the supply, return, and check valve line?
    Edit: Bypassed the fuel selector valve, still over an inch of air in the plastic cup when running to 4450rpm max obtainable rpm
    Click image for larger version. 

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    Last edited by waterpest; 07-21-2019 at 12:27 PM.

  4. #24

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    Can I replace the Pentagon pump with the mikuni triple outlet round pump? Click image for larger version. 

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  5. #25
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    If you don't get a woosh of air when you remove the gas cap in warm weather, I would think you're not building up pressure inside the tank. Cracked tank, bad gasket seal, bad valves, etc...

    You should not be able to blow through the pressure relief valve. It's suppose to open around 2 PSI

    The sender is marked for hose installation. RES is your supply RET is return and vent is self explained

    You could replace the pentagon fuel pump with the round one, but you would be taking a step backwards. The pentagon pump has a much higher output VS. the round one.

    If you suspect the pentagon pump is bad, buy a rebuild kit for it. They're not expensive and quite easy to install.

  6. #26

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    When I receive my mityvac this week I will be able to pressure test the complete system. The check valve and pressure relief valve both tested okay by blowing through them in the appropriate Direction but I realize that is not an accurate test. I don't suspect the fuel pump is defective but I will order a kit and rebuild it just to rule that out but I do believe the fuel tank is not pressurizing. If this is the case, and the fuel in the bowl is just above the bottom of the metal filter, would this cause the engine to lose over 2 thousand RPM?

  7. #27
    Sorry guys I have not been able to get any work done since my last posting due to family matters! And won’t be able work on it for a few more days. I have picked up another o-ring for the separator but have not had a chance to swap it in or vacuum test the lines & tank.

    Waterpest, definitely let me know if u figure out your problem! Only difference is my tank builds pressure when sitting in the heat and I get a whoosh of air out the cap.

  8. #28
    sorry was trying to ask a question of my own I don't know how to remove or delete responses.
    Last edited by Plainfield; 07-22-2019 at 03:54 PM.

  9. #29

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    Good advice, thx...I cancelled my triple outlet round pump order and ordered a Pentagon Fuel pump rebuild kit.

    Quote Originally Posted by xlint89 View Post
    If you don't get a woosh of air when you remove the gas cap in warm weather, I would think you're not building up pressure inside the tank. Cracked tank, bad gasket seal, bad valves, etc...

    You should not be able to blow through the pressure relief valve. It's suppose to open around 2 PSI

    The sender is marked for hose installation. RES is your supply RET is return and vent is self explained

    You could replace the pentagon fuel pump with the round one, but you would be taking a step backwards. The pentagon pump has a much higher output VS. the round one.

    If you suspect the pentagon pump is bad, buy a rebuild kit for it. They're not expensive and quite easy to install.

  10. #30

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    Yesterday I had rechecked and tightened all the fuel clamps from the pump back to the tank, including the vent hose clamps. I also bypassed the Fuel selector switch with a 1/4" brass barbed fitting. So this morning I was still at the Lake, I checked the Fuel Separator and lo and behold the bowl was almost filled to the top after sitting overnight. There was maybe 1/4" of air up top. Jumped on the 780 and ripped around for 10 minutes with NO loss of RPM...I was hitting 6240 rpm and it was pulling hard, like it should. Can I attribute that change in performance from maximum 4700/5000 rpm to the full 6240 rpm to a full fuel bowl??. How would it fill overnight??
    After that 10 minute rip I rechecked the fuel bowl and found about 3/4" of air in it, the filter was about half submerged...had to leave for work, I will continue troubleshooting for an air leak somewhere when I get my MightyVac and can return to the lake...probably on the weekend. Any thoughts??

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