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  1. #1

    GP760 Carb Problems (POP-OFF Pressure)

    I just taken in a 760 that was cutting out when flooring throttle.
    It will not start again unless you use the choke (suspect fuel starvation). I immediately suspected carb problems. I can feather the throttle up gradually and after approx half way it is fine. If i relaese the throttle and allow it to slow and them floor again it cuts out.

    I stripped open the carbs and found the inside all gunked up. Removed everything and cleaned thoroughtly, installed new rebuld kits including neeedles and seats. Problem still exists. Compression is perfect on both cylinders. Fuel selector is perfect and new fuel filter installed.
    Should also mention ski has only 17 hours up but was idle for a long time.

    One thing i need clarified is in relation to pop-off pressure.(see pop off table here http://www.seadoosource.com/mikunitests.html) The carbs on this ski are fitted with 1.5 Needles and seats, I checked the jets and they are as they should be.
    POP off on both carbs is approx 43psi.
    I checked the spring fitted and it looks to me to be shiny silver, but if that is the case the pop off should only be 32psi. Can anyone please confirm the correct pop-off for a GP760.
    I also fitted the 2 new silver springs supplied with the rebuild kit just to check pop-off, one gave 32psi and the other was 43psi. The 43psi one to me was the shiny silver one. But according to the pop table a shiny silver should only give 32psi.

    Pop off gauge has also been calibrated just incase.


  2. #2
    Moderator OsideBill's Avatar
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    Quote Originally Posted by steelroe View Post
    I just taken in a 760 that was cutting out when flooring throttle.
    It will not start again unless you use the choke (suspect fuel starvation). I immediately suspected carb problems. I can feather the throttle up gradually and after approx half way it is fine. If i relaese the throttle and allow it to slow and them floor again it cuts out.

    I stripped open the carbs and found the inside all gunked up. Removed everything and cleaned thoroughtly, installed new rebuld kits including neeedles and seats. Problem still exists. Compression is perfect on both cylinders. Fuel selector is perfect and new fuel filter installed.
    Should also mention ski has only 17 hours up but was idle for a long time.

    One thing i need clarified is in relation to pop-off pressure.(see pop off table here http://www.seadoosource.com/mikunitests.html) The carbs on this ski are fitted with 1.5 Needles and seats, I checked the jets and they are as they should be.
    POP off on both carbs is approx 43psi.
    I checked the spring fitted and it looks to me to be shiny silver, but if that is the case the pop off should only be 32psi. Can anyone please confirm the correct pop-off for a GP760.
    I also fitted the 2 new silver springs supplied with the rebuild kit just to check pop-off, one gave 32psi and the other was 43psi. The 43psi one to me was the shiny silver one. But according to the pop table a shiny silver should only give 32psi.

    Pop off gauge has also been calibrated just incase.
    The pop off you state (43 PSI) is a dull siver 95 gr spring and 1.5 N&S combo, The 760 should have a gold 115 gr spring and 1.5 N&S that is factory spec. 55 PSI pop-off download the manual for specs on Jetting and turns on the adjuster screws.

  3. #3
    Thanks for the reply bill. These carbs are stock and have never been worked on from the factory.

    The spring fitted is a very shiny silver, and that is what is confusing me.
    I fitted each one of the new silver springs in turn and checked the pop-off. I got 32 and 43 psi, 43psi from the longer of the 2 springs, which is the one that I would consider shiny silver by looking at it.
    Manual does not mention pop-off.
    Which one of the silver springs is the longer of the 2.

    I have never seen a 760 fitted with gold spring on this side of the atlantic.
    I seem to be getting 43psi from a shiny silver spring!!!!!

  4. #4
    I removed the springs again and checked, they are silver, and wound clockwise, therefore by refereing to the pop-off table they are shiny silver 65gm. but this does not tally with the pressure I am getting of 43psi. (shiny silver should be 32psi with 1.5 N+S)

    I will fit gold springs as you said and see what outcome is. How the hell did these silver springs get in there?

  5. #5
    Bill found your factory popoff value but Yami spring colors won't jive with mikuni In my experience however your 43 popoff doesn't sound like you running issue to me

  6. #6
    Moderator OsideBill's Avatar
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    I just rebuilt a set of carbs for a member they were from a 760 They had 1.5 N&S, Gold 115gr spring, 115 pilot 1-3/4 turns out, 135 main front, 137.5 main rear 1/2 turn out. The Mikuni rebuild kits for this came with new gold springs also. The manual does not show Pop-off or spring just needle valve size. I inspect pop-off before dissasembly as a baseline and one valve still poped clean at 56 PSI well within spec for the N&S/spring combination. I set this all to factory spec and he replied this week that it hit hard and ran better than before. I have not done too many of these, so I have not read up on differences between years and model's of 760's.

  7. #7
    I fitted the gold springs and checked pop-off, both around 55psi.

    Fitted carbs and prob is still there. If i floor the throttle it revs up, if I leave it idle for a few seconds and then floor it it will choke or die.
    I noticed the fuel filter empties and never fills fully (this is new filter). I checked operation of the fuel valve and this was fine. I blew back all pipes into the tank and all was fine. Checked operation of the one way valve on the tank and all was fine.

    I just remembered that I rebuilt carbs off a GP760 some time back and they were fitted with gold springs.

    Bill, the manual I have specifies everything as you listed above, same N+S and same jets sizes. So I am going to assume the correct springs are now fitted but either way I still have a prob. either it is too lean or the carb is flooding on idle. Needle and seat are new and sealing perfect.

    Any other suggestions would be appreciated.

  8. #8
    Moderator OsideBill's Avatar
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    Pull the fuel sending unit out of the tank and check the flapper door and screen on the bottom of it. It may be warped and restricting flow into your fuel pick up tubes.

  9. #9
    Ok this thing is driving me insane!!

    Went back to basics and re-checked everything.
    Found and checked the following.

    Starting at fuel tank.
    (fuel sender unit not yet checked, next port of call)
    Checked lines from tank to fuel valve - all ok
    Checked Vent valve on top of tank - operating correctly.
    Checked fuel stop valve operation - working perfectly
    New Fuel filter Fitted.
    Checked Pulse Lines - both ok

    Removed carbs to find the following. All of the rebuild kit is fitted correctly. Found one of the new check valves on the front carb fuel pump damaged. Creased on 2 corners (this was the inlet check valve) . Removed it and replaced with new one. Put everything back in place, removed all fuel from tank and replaced with fresh fuel.

    Fire up to observe the following. The fuel filter is emptying quickly. The petrol is not flowing fast enough into the filter. Now either there is a problem in the fuel sender unit or there is not enough suction power from the carb pumps.

    At this stage any pointers are appreciated. I have repaired hundreds of ski's and never had a prob like this before.

  10. #10
    Moderator OsideBill's Avatar
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    Three things

    1. Check the fuel sending unit, if the filter is emtying you have an obstruction somewhere. Personaly I eliminate the shutoff valve and run direst through the filter to the carbs. (do a search on dual fuel supply)
    2. Drill the returns in the carbs out 1/8" (3mm) and install a restrictor jet (95 main jet works best on 1200's not sure on 760's) in the return to tank line, this balances the fuel pumps. You can thread this into the tank at the return or in an brass hose splice it is a 5mm thread. The goal is to create even pressure in both fuel pumps, desired is 1-2 psi at idle,5-7 psi at full throttle. (do a search i for this mod, I am away from home and don't have pics to show)
    3. Double up on the check valve it will help keep it from creasing use an old one on top and the new one for sealing.

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