No rpm change...
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No rpm change...

  1. #1
    Member dirtslinger71's Avatar
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    Default No rpm change...

    My jet boat boat turned 5600 rpm with an A cut impeller. I Changed the cam to more lift and duration... still 5600 rpm. Changed impeller to a B cut still 5600 rpm. My thought is that I may need more carb? It doesn't fall on it face or nose over... just levels out at 5600. I have dual 660's on tunnel ram. Any thoughts would help. Thanks
    ​"use it up,wear it out,make do or do without"

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    Just Me snoc653's Avatar
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    Does it flash higher than 5600 and then settle at 5600? If not does it have a rev limiter?
    So many projects, so little time

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    Senior Member ol guy's Avatar
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    what about valve springs??? Also how old are they??? They are the engines built in rev limiter.

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  6. #4
    Member dirtslinger71's Avatar
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    It will flash a little higher than that. There is no rev chip in my box. Valve springs were checked a year ago and were fine.
    ​"use it up,wear it out,make do or do without"

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    Senior Member jetjunky's Avatar
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    verify your tach's accuracy.

  8. #6
    Member dirtslinger71's Avatar
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    I know it doesn't mean much but it's a pretty new autometer. I'm pretty confident in the Tech's accuracy.
    ​"use it up,wear it out,make do or do without"

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    Some guy obnoxious001's Avatar
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    5600 is high for an A cut impeller, unless you have lots of power, that does not make sense. Any honest feel for how fast the boat is going?

  10. #8
    Senior Member jetjunky's Avatar
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    It's a process of elimination and that's a simple, small step in the process. Unfortunately new/newish doesn't always mean what we want it to or used to. My purchased new/newish autometer tach was off by 200 rpm when verified with a quality instrument, that's why I mentioned it. Many people don't like elec. guages for that reason, inaccuracy.... temps, presures etc. It's your situation to learn as you like, just offering a potential element. Good luck with your deal.

  11. #9
    Just Me snoc653's Avatar
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    Quote Originally Posted by dirtslinger71 View Post
    It will flash a little higher than that. There is no rev chip in my box. Valve springs were checked a year ago and were fine.
    Quote Originally Posted by obnoxious001 View Post
    5600 is high for an A cut impeller, unless you have lots of power, that does not make sense. Any honest feel for how fast the boat is going?
    The fact that it flashes higher means it is probably running out of power. As Obnoxious001 stated that is awfully high RPM for an A cut impeller. Most likely your A was worn and the new B just happens to perform like the worn A.

    More carb might help. But it is sounding like you definitely need more power. High RPM is not always what you want with an impeller. It could be that you need to back to an A or a AA and generate more power lower in the RPM range. That is assuming you are after more speed and not just more RPM.
    So many projects, so little time

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    Member dirtslinger71's Avatar
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    The motor was built with the intention to be 600 hp and turn 6000-6500rpm. I can't even get to were the cam makes power. I might add that with my new B I also added an inducer. The boat is way quicker from a to B but still tops out the same. Have not gps'd it yet but seat of the pants and running beside my dad's boat it's same top speed...
    ​"use it up,wear it out,make do or do without"

  13. #11
    Member dirtslinger71's Avatar
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    Thank you guys for all your help. I was just running out of ideas and figured carbs were my last option... alot of money to throw at it for just a guess.
    ​"use it up,wear it out,make do or do without"

  14. #12
    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by dirtslinger71 View Post
    The motor was built with the intention to be 600 hp and turn 6000-6500rpm. I can't even get to were the cam makes power. I might add that with my new B I also added an inducer. The boat is way quicker from a to B but still tops out the same. Have not gps'd it yet but seat of the pants and running beside my dad's boat it's same top speed...
    What's the engine combo?

    Make
    CID
    Compression Ratio
    Heads
    Intake Manifold
    Exhaust Manifolds
    Carb Model/Type, CFM
    Old Cam Specs
    New Cam Specs
    Springs, if unknown, what did they test out at

    EDIT:

    Found this in another thread...
    Quote Originally Posted by dirtslinger71 View Post
    OK guys I have a 30 over 454 11.5:1 aluminum 310 ported heads,solid roller .660 lift 245 intake and 253 exhaust duration cam.tunnel ram 660 carbs. Cam says 4500-6500 rpm. Jet boat levels out at 5700 rpm with b impeller. Cam is in at 109*advanced...
    Parts selection seems okay, plenty of carb to get there. Just off the top of my head, to get 600 out of your 454 you're gonna have to spin it... If your jet is a Berkeley, I'd be looking at cutting that "B" to a "C"



    Last edited by Budweiser; 06-15-2016 at 11:46 AM.

  15. #13
    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by snoc653 View Post
    The fact that it flashes higher means it is probably running out of power. As Obnoxious001 stated that is awfully high RPM for an A cut impeller. Most likely your A was worn and the new B just happens to perform like the worn A.
    This could very well be the case.

    Quote Originally Posted by snoc653 View Post
    More carb might help. But it is sounding like you definitely need more power. High RPM is not always what you want with an impeller. It could be that you need to back to an A or a AA and generate more power lower in the RPM range. That is assuming you are after more speed and not just more RPM.
    Given the engine combo, last thing I'd do is go bigger.

  16. #14
    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by dirtslinger71 View Post
    ...figured carbs were my last option...
    A pair of 660's isn't a restriction in your case. Don't pitch more cash that direction.

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