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  1. #1

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    FrankenSki - 15F Motor in Ultra Hull

    I have a 2006 STX-15F motor (built from parts from a few different motors), the ECU/gauges/harness/pump from a 2007 Ultra LX and a 2008 Ultra 250X hull... all spare parts laying around the house.

    I'd like to combine all of this into a working JetSki. For the most part, it's a fit. Move the motor mounts back. Plug the thru-hull ports that supply the intercooler. Swap the gauges, harness, ECU and pump. Drop in motor. Ride.

    The one issue I can see is the pulsing rotor on the front of the crankshaft. The Ultra LX uses a completely different part than the 15F, but the Ultra LX pulsing rotor won't fit on the 15F crank, as it's made to fit the smaller diameter of the end of the newer crank. See the photos of the 2 parts (15F on left). The Ultra LX rotor has a thicker ring on the back, which reduces the inside diameter.

    I'm guessing the Ultra LX ECU won't run the motor with the wrong pulsing rotor. Does anybody know if it will?

    Is there a pulsing rotor, either OEM or A/M that will fit this crank and be compatible with the Ultra LX ECU?

    If I modify the Ultra LX rotor to fit the 15F crank, will this motor run properly, or are there other issues that I'm missing?







    It's Alive... IT'S ALIVE!
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    Last edited by grumpy_steven; 05-06-2019 at 06:21 PM.

  2. #2

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    Looks like the Dremel did the trick.



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  3. #3
    Ultra 15f's Avatar
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    I used a 2008 250x rotor on a 2004 STX 15f. It fit without any modifications


  4. #4

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    Are you sure it was the original 2004 motor? I measured the ends with a micrometer, and the newer crank is definitely smaller.

    I have a few cranks here. In addition to this one, I have a 12F crank and a newer crank. The most noticeable difference is that the older cranks use a Woodruff key for the flywheel. The newer cranks do not.

    Also, the different pulse rotors must send significantly different pulses. I'll bet the ECU wouldn't know what to do.
    Last edited by grumpy_steven; 05-06-2019 at 09:23 PM.

  5. #5
    Bucky STX-15F's Avatar
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    nice work, keep us informed on the progress.

  6. #6

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    Almost done... waterbox and intake manifold.

    Have to address the issue of the smaller exhaust outlet pipe on the 250X.

    Spent the day helping a friend with his ski, so nothing got done.

  7. #7

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    Motor is in and running.

    Still have to address the exhaust outlet pipe. For now, the diameter was built up with the universal fix-all... duct tape.

    Next, I'll install a pump. Thinking about using the Ultra 250 pump (upgraded shaft) and installing the intercooler. I have two intercoolers, and it's possible that a leaking I/C was the cause of one or both of these engine failures. This would be the easiest way to find out whether they are good. And I'm tempted to rig up something to send the cool air into the intake... an intercooled Ultra LX???

  8. #8

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    Biggest issue was the exhaust outlet pipe. The Ultra 250 pipe was the correct length, but the 1.75" diameter was too small. The STX-15F pipe was the correct 2.25" diameter, but too short for the bigger hull.

    I remembered that the 1100ZXi exhaust has a metal coupling between the expansion pipe and the waterbox. Dug one up and it was the correct length and diameter to extend the 15F pipe. Cut the 15F pipe and used the hoses and clamps that came from the ZXi to connect the 2 pieces. The 3 pieces of pipe are all butted against each other, so the hoses shouldn't see too much action. If this setup works, maybe I'll get the pieces of pipe welded together. For now, this should do the job of getting the exhaust out of the hull.






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  9. #9

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    Last thing to do is install the airbox. It looks like the Ultra LX uses a different bracket, so I'll have to search for one.

    I used the Ultra 250 jet pump, so the intercooler line is being fed by the pump. I'm going to install the intercooler, initially just to make sure it doesn't leak. Then I might look into routing the cooled air directly into the throttle body... an intercooled Ultra LX.

    Some pics of the motor:



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  10. #10

    Join Date
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    If you need those pipes tigged. Send them over the pond to CT. and I will do it for you and pressure test.

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