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new jet guy needing help

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    Default new jet guy needing help

    i bought my first jet last year and love it. i have always had v-drives and this thing is just sooo much fun to drive. here is my problem. its a spectra 20 with a new hardin marine 460. has a 750 double on top. jet is the 12jc with a diverter on it. im only getting about 4k out of the engine and about 45mph. what can i do to help this. im thinkin of pulling the engine to re cam it but how do i know what impeller i have without tearing it down . it has had pump work but i dont know what . any help would be great

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    Senior Member GT Jets's Avatar
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    Quote Originally Posted by sangerfan76 View Post
    i bought my first jet last year and love it. i have always had v-drives and this thing is just sooo much fun to drive. here is my problem. its a spectra 20 with a new hardin marine 460. has a 750 double on top. jet is the 12jc with a diverter on it. im only getting about 4k out of the engine and about 45mph. what can i do to help this. im thinkin of pulling the engine to re cam it but how do i know what impeller i have without tearing it down . it has had pump work but i dont know what . any help would be great
    IMHO there is something really wrong with the motor not being able to pull past 4K, even if it had a AA impeller it should be able to pull 4.5K if the tune up was even close, I would look for something obvious first, not convinced you need mods as much as a good tune...Just my .02c

    What altitude are you running, and how well does the engine seem to run, this should be a pretty long list of possibilities so you will need to narrow it down...

    GT
    GT


    Quote Originally Posted by Quickjet View Post
    Put a 300 on the back of it, Flywheel it and a nosecone. $15,000 later you'll have a 65 mph pile of shit......

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    Quote Originally Posted by sangerfan76 View Post
    i bought my first jet last year and love it. i have always had v-drives and this thing is just sooo much fun to drive. here is my problem. its a spectra 20 with a new hardin marine 460. has a 750 double on top. jet is the 12jc with a diverter on it. im only getting about 4k out of the engine and about 45mph. what can i do to help this. im thinkin of pulling the engine to re cam it but how do i know what impeller i have without tearing it down . it has had pump work but i dont know what . any help would be great
    Now tell me, how does it get up on plane ? Does the engine rpm flash higher and then fall back to 4k ? Does it have a locked out dist. If so where is the timing set ? If not and it has advancing weights, where is the timing set ? Can you post a picture of the engine ?

    Jim
    Jim & Amanda Rich
    CJ 72 & 53c

    JR Performance Motorsports

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    Just a thought, but is the carb opening 100%.


    Darrell.
    San Diego.

    28ft Sportcat
    With Twins

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    Senior Member Sanger Pete's Avatar
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    SF
    You could also check fuel pressure.
    Sanger Pete

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    Senior Member bp298's Avatar
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    Quote Originally Posted by sangerfan76 View Post
    i bought my first jet last year and love it. i have always had v-drives and this thing is just sooo much fun to drive. here is my problem. its a spectra 20 with a new hardin marine 460. has a 750 double on top. jet is the 12jc with a diverter on it. im only getting about 4k out of the engine and about 45mph. what can i do to help this. im thinkin of pulling the engine to re cam it but how do i know what impeller i have without tearing it down . it has had pump work but i dont know what . any help would be great
    you can't know what impeller it has without tearing it down. depending what's been done to the pump, what impeller it might have installed, it's not all that surprising that the engine won't exceed much more than 4k. my 23' hallett junkyard truck 454 will only get up to 4100, but the boat will get over 50 with it, but i know what's in the pump, so 4100 for this little engine isn't surprising.
    you have a 750 holley, but is it mech secondary or vacuum secondary? if it's vac, the secondary's may not be fully opening.

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    i am pretty close to sea level a few hundred feet. it is a 750 dbl pump holly with 75 jets in it. the ignition is a mechanical advance im not quite sure the setting i forgot. as far as the pump ill check the clean out. the boat does ok out of the hole and great mid range but wot just stops and the carb is opening wide open. ill check the filter setting for the dist. i do notice it can be hard starting at times. im working the probs 1 by 1. the guy i bought it from only had it a few weeks and he got it from the orig owner . boast clean and love it but i know it can do better. if not the blue oval comes out and the bow tie goes in .

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    You'll get it figured out. And don't worry, those 460 are good motors, and can make some serious hp with the right parts. No need to yank her out.


    Darrell.
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    Senior Member NO BUTT NO PUTT's Avatar
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    Check your tach first.

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    Senior Member Delemorte's Avatar
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    Quote Originally Posted by sangerfan76 View Post
    but how do i know what impeller i have without tearing it down .
    Yank the bowl and check the stamp. Its not hard. I yanked mine for the first time recently and that was the easy part.. Just dont get sucked into the rebuild unless you are prepaired for it... I went in to replace some nylar bushings and rebuilt the pump yanked the motor laid new carpet and re-ran all the plumbing and eletrical... Have fun!!
    Quote Originally Posted by jetboatperformance View Post
    the ensueing fire would likely be extinguished by the sinking however

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    fuel and timing work togther but in my exsperiance, run wot watching tach and listening, back off the throttle just alittle if it picks up rpms alittle then up the jet size. also make sure you have enough fuel pump and large enought lines and fittings no restiction. also chech fuel filter for SHORT RUN ONLY I have bypast the filter Never ran a blue oval but am running a 750 doublpumper on a 350 bow tie with 82 jets 5200 rpms still looking for more. Hope maybe this helps

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    Many boat motors have been wasted and replaced with car motors. Boat motors came with larger cams,carbs,different dist curves,and in some cases,bigger valves too. A car engine in a jet usually runs 4200 rpm and 42-45 mph. Before you stuff a cam, etc. in it, make sure it has a big oil pan, at least 8qts, 10-12 is better. TIMINATOR
    MODESTY IS A CRUTCH FOR THE INCOMPETENT!

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    Senior Member bp298's Avatar
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    Quote Originally Posted by sangerfan76 View Post
    i am pretty close to sea level a few hundred feet. it is a 750 dbl pump holly with 75 jets in it. the ignition is a mechanical advance im not quite sure the setting i forgot. as far as the pump ill check the clean out. the boat does ok out of the hole and great mid range but wot just stops and the carb is opening wide open. ill check the filter setting for the dist. i do notice it can be hard starting at times. im working the probs 1 by 1. the guy i bought it from only had it a few weeks and he got it from the orig owner . boast clean and love it but i know it can do better. if not the blue oval comes out and the bow tie goes in .
    yes, it probably could do better, but how much are you willing to spend to get it there? how much better do you believe it will do (i.e., at what point would you be satisfied)? spectra's are heavy layups, and yours seems to be running pretty much the way it should for what it is. you could take that thing out in the ocean and jump waves with it, and it won't hurt it.
    there's always more power and setup items to try. but you just got this thing. why not just keep identifying and correcting little detail items like you're doing, use it for the season while making a nice list of things to do during the next off season?

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    when working the throttle i can tell there is more in the engine , it doesnt have that full throttle sound

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