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  1. #1

    slt 780 idle..... normal?

    Got my 97 slt 780 out on the water last weekends and it ran like a champ 34 hours on it so pretty new. Up on plain didn't skip a beat. Low throttle smooth as silk, take off nice and powerful. Idle sounded like it wanted to throw a piston out the top of the jug " figuratively speaking" real erratic and not smooth at all. No throttle play and the rpm gauge read 1180 lowest 1230 highest during the idle and stayed constant with that all weekend just curious is this normal? I expected smoothness and there's clunks bumps knocks here and there. No care to me unless something needs to be fixed


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Have you checked cylinder compression across all three cylinders?

    What is the maximum sustained RPM you are seeing at full throttle?

    Correct spark plugs?

    Did you remove the spark plug caps from the ignition wires, trim the wire ends a few mm, then screw the spark plug caps back on the wires.

  3. #3
    BlueFishCrisis's Avatar
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    Quote Originally Posted by K447 View Post
    Have you checked cylinder compression across all three cylinders?

    What is the maximum sustained RPM you are seeing at full throttle?

    Correct spark plugs?

    Did you remove the spark plug caps from the ignition wires, trim the wire ends a few mm, then screw the spark plug caps back on the wires.
    +1 - Check all these. I like to idle closer to 1500. It helps to get rid of the apparent "roughness".....

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    Factory spec is 1200-1300. Maybe bump it up just a touch

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    john zigler's Avatar
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    Could also be hearing your driveline. Your shaft should have bumpers on each end, and "IF" they are missing / worn you will hear a knock at idle until the pump loads.

    I agree though with above. Change all plugs, cut wires back 1/4 inch etc first.

  6. #6
    Compression is 130 on the back 2 cylinders and 135 on the front. I haven't trimmed any spark plug wires. How can I tell if the previous owner did? And at WOT I haven't done that for very long but with 340 lbs on the ski we seen 50mph and I think I seen 5200 -5300 on the rpm gauge. Is there an idle adjustment for all the carbs or do you have to bump them up individually. I can google how just curious if there's one screw or three?

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    BlueFishCrisis's Avatar
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    A freshly rebuilt 780 comes in at 140 psi on all three, so I don't think you are too far off. Go ahead and trim the wires either way. Won't hurt to take another 1/4" off (as long as the wires still reach that is). BPR8ES plugs.

    There is one large T handle at the PTO (back) end of the carb rack that sets the idle speed. Adjust the idle speed in the water. Should max out around 6500 +/- RPM and around 50+ MPH. You probably saw 6200- 6300 at that speed.....

  8. #8
    Found the t handle thanks, come to think of it i only took it to the limit once and i had the mph dialed in not rpm so i don't know rpm wise where it was at the top speed. I'm taking it to the lake again this weekend so I'll bump up the idle see if it smooths out the idle I'm sure its going to. Also i think i am hearing the drive line bumping cause the knocking is not consistant with rpm more like a pop corn machine rhythm. Is it really bad for them to run with those worn bumpers for the drive line not. Inside the hull its brand new everything. In the jet drive I'm not sure.

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Un-cowJ View Post
    ... the knocking is not consistant with rpm more like a pop corn machine rhythm.

    Is it really bad for them to run with those worn bumpers for the drive line not. ?

    Inside the hull its brand new everything. In the jet drive I'm not sure.
    It is still worthwhile to trim back the spark plug wires a few mm. No cost and easy to do.

    Also verify you have the correct NGK resistor spark plugs installed.

    I suggest you remove the jet pump and inspect. Damaged or missing driveshaft end bumpers can be replaced short term with plumbing flat washers, just use the correct thickness.

    While the jet pump is out check the pump bearings for smooth operation. The impeller should turn with zero noise, no grittiness or slop.

    Inspect the drive shaft ends for spline damage. Also check the mating splines on the engine coupler and the impeller. Make sure the drive shaft is straight and is not grooved or damaged where it rides inside the through-hull bearing.

    Make sure the through-hull bearing is snug on the driveshaft. Wobble or play between shaft and through-hull assembly says the bushings inside are worn.

    When you put it back together, squirt some waterproof marine grease into the spline cavity on the engine coupler and also into the impeller spline cavity. Make sure the bumpers stay in position on each end as you slide the pieces together.

    Ignore the speedometer, pay attention to the tachometer. RPM is where the information is.
    Last edited by K447; 03-21-2014 at 07:52 AM.

  10. #10
    BlueFishCrisis's Avatar
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    Quote Originally Posted by K447 View Post
    check the mating splines on tepee

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