jet pump question
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jet pump question

  1. #1
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    Default jet pump question

    I was told something real wierd about my jet pump it has a little viberation in it i called a shop and he told me the motor has to be a little offset from the driveshaft to make it stop. I have had all v-drives this is my first jet and i have always lined the motor up the best i could with the v-drive. Sure sounds like bull to me wanted a few other thought on this.

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    Senior Member Luckie Stiff's Avatar
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    It's true, your crank and pump shaft should have 1.5* to 3* difference between them, when I set my motor and pump up I set the motor 1/2" higher @ the flywheel, this gave me about 1.25* deflection and I'm ok with that. I know it doesn't make any sense but for some odd reason the U-Joint wants deflection, it's the same on cars and trucks.
    "Don't you realize that there are already enough people in the world to hate without you putting in so much effort to give me another?"

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    Default jet pump

    Alright just wanted to make sure sounded a little crazy thats what i am going to do then. Thanks for you help

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    Sit N' Spin Jetaholic's Avatar
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    From what I've been told the H-bar has to have some offset in it to keep the needle bearings in the u-joints moving.

    Basically the pump shaft and the crankshaft have to be exactly parallel to each other +/- 1-3* but with a little bit of offset in the H-bar (crankshaft on a slightly higher plane than the pump shaft).

    If it was meant to be perfectly straight (i.e. 0 offset) there would be no purpose of using a double cardan joint and we'd all use a straight shaft.



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    B1 Racing cs19's Avatar
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    is the vibration a new thing?
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    Senior Member Oldsquirt's Avatar
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    Quote Originally Posted by Jetaholic View Post
    If it was meant to be perfectly straight (i.e. 0 offset) there would be no purpose of using a double cardan joint and we'd all use a straight shaft.

    Jon, the H-bar shaft used in jet boat applications is NOT a "double cardan joint". Although a typical H-bar can be made using some of the same components(end yokes and center "H"), they do not use the interconnected u-joints that identify the double cardan joint. The double cardan joint would not allow the shaft centerlines to be parallel with an offset.

  9. #7
    Senior Member wizard612's Avatar
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    Without re-rigging your motor you might want to consider what we used in our old circle boats back in the day. A Lenco driveline. They have to be straight no "bends". I also liked it because with an allen wrench you can disconnect your motor from the pump in 5 seconds and run the motor without spinning the pump. Less wear on a dry pump or it won't get anyone behind the boat wet if there is water in the pump. Only problem is you have to remember to hook it back up before you launch ( don't ask me how I know this). It is a bit noisier since there needs to have a bit of play in it so you don't damage your bearings if the motor / pump flexes.

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    Quote Originally Posted by cs19 View Post
    is the vibration a new thing?
    I agree with CS19. Did the vibration just start? Maybe something in your pump.

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    Default jet pump

    No its not new its been this way every since i put the motor in it

  12. #10
    Senior Member H20MOFO's Avatar
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    I assume you've ran it without the driveline and it goes away? U joints maybe?
    Another Hot Boat refugee

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    Senior Member GT Jets's Avatar
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    Very common issue, I have had a constant velocity (CV) joint made years ago for a jet that worked pretty well, but found it to be completely unneccissary, if you put some vertical and even misalignment in the DL, your noise should vanish, on the double joint DL, I always line the DL up dead nuts and raise all four corners about an 1/8", this loads both joints the same and eliminates any chance of harmonics.

    Also make sure the yoke does not bottom out on the pump shaft, this will cause a similar noise and destroy the rear main bearing thrust surface forcing an early engine overhaul...There should be at least 3/16" of space between the end of the pumpshaft and the bottom of the yoke IMHO.

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    Last edited by GT Jets; 02-17-2009 at 06:44 PM.
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    I looked at one of my buddys jet boat today his is ofset to the right just a little bit would that work out for me

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    I bet if you google u-joints and dig in deep enough you will eventually learn something along the lines of this:

    If your H-bar/drive line alignment is perfect, then the bearings in the u-joint have the opportunity to kind of float around, or at least stop spinning the needles. This sets up the possibility for harmonics to creep in and start banging the needle bearings. If they're not spinning then they can get a flat spot quicker and then, the next thing you know you're having a bad u-joint day.

    There are two types of ways to intentionally (or accidentally) mis-align your u-joints. 1) Parallel shaft but slightly offset and, 2) crooked shafts.

    It's been awhile since I looked but I think the drive shaft rule of thumb is to have the crank centerline and the pump shaft centerline parallel but slightly offset. In reality this is harder to rig than just going for the crooked scenario. Like everyone is saying, just line up everything perfect and then intentionally shift the motor crank about 1-3* crooked and you will probably acheive the best you can toward "flawed perfection".

  16. #14
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    Default jet pump

    You guys have helped out alot thanks for all the help

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