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  1. #1
    20 feet Boy's Avatar
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    Seadoo 720 trouble starting

    Does anyone have any ideas on what the issue could be with my 720 Seadoo randomly not starting. It starts up fine sometimes and the after being on the lake for a bit it doesn't restart - as if it is starved for fuel.

    The compresssion in the Mag side is 100 lbs and the compression in the PTO side is 125 lbs. The motor has 15 - 20 hours on it and the cylinders look good. Could the lower compression on the one side make the fuel pump not pump as well?

    If the rotary valve is not in the right degree or off a tooth, could that be an issue?

    Thanks.

    20 feet Boy


  2. #2
    JFizzleJR's Avatar
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    maybe oil build-up in the cylinders?

  3. #3
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    Quote Originally Posted by 20 feet Boy View Post
    Does anyone have any ideas on what the issue could be with my 720 Seadoo randomly not starting. It starts up fine sometimes and the after being on the lake for a bit it doesn't restart - as if it is starved for fuel.

    The compresssion in the Mag side is 100 lbs and the compression in the PTO side is 125 lbs. The motor has 15 - 20 hours on it and the cylinders look good. Could the lower compression on the one side make the fuel pump not pump as well?
    kinda on the low side, you should see something around 150. and no this will not affect the pulse on the carbs unless you have a leak on that pulse line, or a blockage.

    If the rotary valve is not in the right degree or off a tooth, could that be an issue?
    but how can it be off???? did you take it off in the past? it wont jump a tooth by itself

    Thanks.

    20 feet Boy
    if you have the grey fuel lines this could be the problem.

  4. #4
    flyin' the friendly skies airbornexp's Avatar
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    your mag compression sounds very low...who rebuilt the motor? you should be with in +/- 5psi on a new rebuild. Is this an SBT reman? you could be experiencing blow by on the cylinder and fouling the plugs. did you break this motor in correctly or did you just hammer down and not look back?

  5. #5
    20 feet Boy's Avatar
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    The motor is a PMC and it went out of business. When I put the rotary valve in it was a first for me. But I did it just like it shows in the diagrams. The break-in was an easy ride for 45 min not keeping throttle over half and moving it around alot. If I have the rotary valve off I was told the motor will sputter and backfire so I think it is right. Will I get blow-by from the rotary valve lowering the compreston in the MAG cylinder? If so, maybe I don't need to ring it. I have a short summer so the motor is only 3 years old. It has always had a hard time starting. But it has been getting worse. I guess I will re-ring it and go from there. Does anyone know what rings are in my motor? Top of piston say .075 and is most likey a cast piston. I have cleaned all filters.

    20 Feet Boy

  6. #6
    20 feet Boy's Avatar
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    What do you think about this?

    Quote Originally Posted by airbornexp View Post
    your mag compression sounds very low...who rebuilt the motor? you should be with in +/- 5psi on a new rebuild. Is this an SBT reman? you could be experiencing blow by on the cylinder and fouling the plugs. did you break this motor in correctly or did you just hammer down and not look back?
    The motor is a PMC and it went out of business. When I put the rotary valve in it was a first for me. But I did it just like it shows in the diagrams. The break-in was an easy ride for 45 min not keeping throttle over half and moving it around alot. If I have the rotary valve off I was told the motor will sputter and backfire so I think it is right. Will I get blow-by from the rotary valve lowering the compreston in the MAG cylinder? If so, maybe I don't need to ring it. I have a short summer so the motor is only 3 years old. It has always had a hard time starting. But it has been getting worse. I guess I will re-ring it and go from there. Does anyone know what rings are in my motor? Top of piston say .075 and is most likey a cast piston. I have cleaned all filters.

  7. #7
    20 feet Boy's Avatar
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    I rip it apart and the lower ring was haft missing. I cant believe the cylinder is not trash's. The piston said 55.1 PMC on the in side and 0.75 on the top. I been looking for a piston to match.No luck,I guess I will re do all. Money pit. PMC sucks no wonder they went out of business. The rotary valve is in the right place. I am putting new USCG Trident fuel line too.
    Last edited by 20 feet Boy; 07-27-2010 at 02:24 PM.

  8. #8
    20 feet Boy's Avatar
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    This thing is still not ruining right

    I got the ski back together and put pro x piston in it. New fuel lines and the rotary valve is in the right place. I broke the ski in runs great. When idling in to shore it started to die. Fanning the throttle it blubber and came out of it. IT started to blubber again when rpm went down. Again fanning throttle blubbering it started to run again and sometime blubber Intel dies. It is hard to start after that but usually will start with some throttle fanning and then blubber out of it to full run. The ski runs fine the rest of the time. Its after it is warm up,is when has the idling problem . The idle is set a little hi because I set it that way for the altitude.Idling out of water sometime is not enough to idle in water. What should I look at next. please help THANKS

  9. #9
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    sounds like its loading up with fuel on the low end. Low speed screw might need to be turned in

  10. #10
    20 feet Boy's Avatar
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    thanks I check it

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