low rpms
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low rpms

  1. #1
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    Default low rpms

    when running on flat water i can only get up to about 4400rpm. my motor(bbf) is not cutting out or anything, could this be because of my impeller? i dont know what cut or blade it is but its in a yj jacuzzi. any ideas. thanks

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    A jet pumop is like running on a dyno or a water-brake. if you can spin only 4,400 RPM, ione of 3 things is the case.
    Motor weak
    Impeller too large
    Combination of Above.

    Bottom line, the pump is absorbing all your engine is making, at 4,400RPM.
    If your engine made more power, it would continue to accelerate, untill the pump demand equaled power output.

    My stock engine spun a Berk A 4,800RPM.
    After I changed cam & Intakes, I also (simultaneously unfortunately) added a top loader to the pump, 4,800RPM.
    Added jacketed headers, 5,000RPM
    Added 100HP Nitrous plate system, 5,400RPM
    I had the heads ported, valves enlarged, did port matching on intake, 5,400RPM.
    With 150HP jets in the nitrous, 5,900 RPM.

    The Tach is your best diagnostic tool.
    Most of the time:
    Sudden unexplained RPM increase = pump wear/damage.
    Sudden unexplained RPM decrease = engine issue/power loss
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    It takes approx 250HP to spin that pump at 4400RPM. So now you know your engine HP.

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    this really does not make sense because i have the motor... its a bbf 460 with 429 cobra jet heads ported with a high rise intake port matched, a big cam, all forged internals, aluminum pistons. the motor prob has 500hp. and it only has about 15 hours on it...

  7. #5
    Senior Member jetboatperformance's Avatar
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    Blain assuming your tach is correct it sounds like your down on power a little , complaints with YJs (that HAVENT been modified) are generally the opposite (High RPMs vs speed). 90% of all YJs I've ever seen over 20 years have a "B" cut impeller and most BBFs will want to spin this jet over 5K WOT . This Jet (type, style ,operation and construction cannot directly be "equated" to Berkely and other similar type jets as it really does work differently. The Yj moves large volumes of water (axial flow) vs creating pressure . I would be looking for things like wide open throttle adjustments , timing issues,altitude ? and fuel supply. Please feel free to call if we can help Tom


    PS. Just as an example My "Pops" bought one of these 76 Omega , Ford YJ setups in a day cruiser back in the day ," Out of the box" it would take 4k to plane it and 5k to run 55 mph.Conversely My new 76 Miller (same motor and aBerk jet) ran 65 @ 4600 rpms (planed at 3200)

  8. #6
    Senior Member Brendellajet's Avatar
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    This is kind of strange because I thought the axial flow pumps spun more RPM than the Mixed Flow. YJ pumps are axial flow(like a panther). Make sure the tach is good.
    "He is a lover, not a fighter. But he's also a fighter, so dont get any ideas."

  9. #7
    NW Spectraholic Malcolm's Avatar
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    We had a tunnel ram on the river a few years ago that was just running on the primaries. Turned out the throttle linkage was only good for half! Check it out.
    Malcolm

    75 Spectra 18
    77 Spectra 19 project

  10. #8
    Senior Member propless's Avatar
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    When I first got my old boat it had a stock 460 with a YJ-A, it would turn around 4500rpm. After the motor was built it would spin it to 6300rpm (still didnt go anywhere ) ). If you realy like your boat, look into an Energizer kit. If I remember right the YJs are only good for up to something like 350hp.

    Got any pics of the pump ? This just doesnt sound right for a YJ. Maybe it already has an Energizer kit on it.

  11. #9
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    ill have to take a look at the carb to make sure the secondaries are opening. thanks for the help guys.

  12. #10
    Senior Member Brendellajet's Avatar
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    Ditto from above, axial flow pumps reach a point where they spin faster and dont go faster. If ou want to go fast, dont bother with the YJ, either switch pumps, or switch boats.
    "He is a lover, not a fighter. But he's also a fighter, so dont get any ideas."

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