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Thread: Very high idle

  1. #1

    Very high idle

    Hi - I finally got the chance to start my Jet Jon, which has a 750SL drive.

    It starts right up but goes to 6000 RPM & stays there.

    Here is the sequence of whay I have done:

    Last fall, it ran & idled OK before I diassembled the jet ski.

    Last winter I rebuilt carbs (Watcon kit) & installed triple out FP.

    I followed the carb rebuild instructions per shop manual to the best of my ability. Sync'd carbs with the drill bit method.

    I set mixture screws to recommended initial settings.

    Put in new carb-to-manifold gaskets, torqued to spec.

    Throttle shaft is against stop.

    The air intake does not have the rubber boot attached.

    BTW, I am testing it without pump attached (no load at all) , have not let it run for more than 15 seconds at a time. Gave it hose water after first start but not again - don't run it long enough to get to the spigot & back.

    My hunch is that I have a intake air leak.

    Once last thing - I changed the oil pump back to orginal variable type. The oil hasn't made it al the way to to the carbs yet ( half way down the clear lines). I am running premix for now. I don't think the air in the lines is the culprit but thought I would mention that.

    No changes were made to ignition system.


    I was wondering if there is any simple explanation for this before I tear it all back apart again.

    Any & all advice much appreciated.

    Cheers


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Does the throttle cable have at least a tiny bit of slack when in the idle position?

    If you remove the throttle cable from the carb rack does the problem go away?

  3. #3
    I released the cable from the lever, but not from the rack. I'll do that next. I am also going to install the pump & back the boat into the water so there is some load.

    Thanks for the advice. The good news is that it starts very easily. Before the carb & FP rebuild (last fall) it was very hard to start.

    Edit - I found this info from another post, hopefully there is nothing wrong! "Yes, the ski will rev high for a few seconds upon starting on the trailer. its ok, it will calm down in a min."
    Last edited by JonJet; 03-11-2015 at 08:25 PM.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by JonJet View Post
    I released the cable from the lever, but not from the rack. I'll do that next. I am also going to install the pump & back the boat into the water so there is some load.

    Thanks for the advice. The good news is that it starts very easily. Before the carb & FP rebuild (last fall) it was very hard to start.

    Edit - I found this info from another post, hopefully there is nothing wrong! "Yes, the ski will rev high for a few seconds upon starting on the trailer. its ok, it will calm down in a min."
    It is normal for the carburetor engines to idle out of the water at roughly double the in-water idle speed. So maybe 2400RPM or a bit faster without load.

    6000 RPM is way too high for idle, even without any load on the engine.


  5. #5
    So I went through all of the pictures I took during my fuel system rebuild. Came across the picture of the autocock, with impulse line. Hmm, maybe I didn't cap that puppy off . . .

    I had to feel for it, can't even see it with a mirror. Sure enough, there is an open elbow.

    I'll get a cap for it today.

    Thanks K!

  6. #6
    BlueFishCrisis's Avatar
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    You should idle around 3k RPM out of the water. Set your idle speed with the adjustor at the end of the rack. Hopefully capping the elbow does the trick!!

  7. #7
    After fumbling around trying to put a cap on a screw head, I figure out there is no elbow there.

    I took the flame arrestor cover off & saw that C & P carbs are slightly more open than M.

    So much for the drill bit sync method.

    I readjusted the 2 rear carbs so they were fully closed but moved same as M carb.

    It now idles at 2700 . Going to put the pump on next, maybe go for a ride yet today.

    Thanks all!

  8. #8
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    i sync my carbs by setting the mag carb blade where it blocks the idle port hole half way when sighted down the blade,then set the cen and then the pto to match,never had an issue.done tens of them this way

  9. #9
    Well, I have sucessfully ran it at ramp on trailer. It idles at 1300, so that seems OK.

    I am getting a stutter about 3500 RPM, I assume this is where it goes to hi speed circuit.

    My guess is the hi speed jets needs to be richer. Any thoughts on that?

    The river has a wicked current & had icebergs floating by, so I got shy about heading out with an unknown engine. Will wait a few weeks to take the shakedown cruise.



    Thanks

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by JonJet View Post
    Well, I have sucessfully ran it at ramp on trailer. It idles at 1300, so that seems OK.

    I am getting a stutter about 3500 RPM, I assume this is where it goes to hi speed circuit.

    My guess is the hi speed jets needs to be richer. Any thoughts on that?

    The river has a wicked current & had icebergs floating by, so I got shy about heading out with an unknown engine. Will wait a few weeks to take the shakedown cruise...
    Was it able to power up to full throttle? I am wondering if it is hitting the electronic RPM limiter.

    Some engine models/CDI will trigger RPM limiting for Low Fuel or a few other triggers. The limit, when engaged, is circa 4500RPM, going from fuzzy memory.

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