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  1. #1

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    Default Newbe here

    have a 16ft jet with a berkelley 12jc AA. Pump was just rebuild. Motor is a 468 stock with hyper 10 to 1 pistons, 540 max lift cam with a lobe center of 108. dual plane intake with a holly 750 dual feed vac sec. took it out for the first time and it is only turning 4600 RPM's. Is there anything worng or not. How can i get the boat to turn more RPM's.

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    21 Daytona Outlaw's Avatar
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    Welcome,
    if it really has an AA impeller 4600 RPM sounds correct to me.

    why do you want more RPM?

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    I just realy want to go a little faster. all my buddies boat do 5200 to 5500. At 4600 it is only doing like 60mph.

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    If i took the AA out and had it cut down to an A or even a A/B cut, what would happen.

  7. #5
    Senior Member NO BUTT NO PUTT's Avatar
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    Quote Originally Posted by jetboatkid View Post
    If i took the AA out and had it cut down to an A or even a A/B cut, what would happen.

    More rpms.

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    Senior Member H20MOFO's Avatar
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    A gps # would help. Where your at now.
    Another Hot Boat refugee

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    Quote Originally Posted by jetboatkid View Post
    I just realy want to go a little faster. all my buddies boat do 5200 to 5500. At 4600 it is only doing like 60mph.
    This will give you an ideal : (maybe)



    In a light 16" boat I wouldn't think twice about an A/B or B cut impeller.

    Do you have any idea of the power the engine is making ?

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    Senior Member fleetimus's Avatar
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    What kind of hull???

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    Glendale Arizona Squirtcha?'s Avatar
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    Quote Originally Posted by jetboatkid View Post
    If i took the AA out and had it cut down to an A or even a A/B cut, what would happen.
    Not necessarily. It depends on what RPM you're making your max horsepower. If you're
    already in the right RPM range for your motor. It's possible that you could end up turning
    more RPM and not gain any speed from it.

    Tough call unless you have a dyno sheet, or you just want to have someone start cutting
    on your impeller in hopes that it will work.

    You could try running your combination on a desktop dyno program, but it would be ball
    park at best.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by fleetimus View Post
    What kind of hull???
    Need more info on the boat!!!!!!!!!!!!! Believe it or not the 18ft boats were more efficient on the water than a 16ft. there were VERY FEW boat builders that made a fast 16. Again need more info!!!!!!!!!!!! M

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    Quote Originally Posted by jetboatkid View Post
    dual plane intake with a holly 750 dual feed vac sec. .
    It might just be me, but I hate Vac secondary carbs. What spring is in it and what did the engine pull before the rebuilt pump ? or is this a new combo for you ?

    Sleeper CP

  14. #12
    Glendale Arizona Squirtcha?'s Avatar
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    Excellent points. Although your numbers sound about right it would be a great idea to make sure that you're in good tune with your current setup.
    Timing, plugs, points (if applicable), carburation/fuel delivery etc.

    I've seen vac sec carbs where for various reasons, the secondaries weren't opening on them. Have someone watch the carb and make sure it's working properly and they're opening under WOT. In the instances I witnessed getting the secondaries to open (facilitating repairs) was good for another 100 rpm or so (consequently more speed followed).

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    This is a new combo for me. The vac sec is opening all the way i had someone look. the boat turn 63 mph on a hand held gps. he is a video of the past. tell what you think i should do. more power or cut the impeller. motor is pushing the 400hp mark

  16. #14
    Sit N' Spin Jetaholic's Avatar
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    More RPM doesn't necessarily mean more MPH. Pump efficiency, hull/hardware setup, all of that comes into play.

    Cutting your impeller down will allow you to turn more RPMs, however you'll be moving less water per RPM than you will with a larger cut. Think of the impeller cut as you would the gear ratios on a bicycle. When you have it in low gear, you can spin the crap out of the bicycle crank, but the bike really doesn't go anywhere. Put it in high gear and spin it at the same RPM you can spin it in low gear and the bike hauls ass. But it takes more power to be able to spin it that high in high gear.

    Personally, I favor going the other route...add more horsepower.



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