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  1. #71
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    There's a proper procedure for syncing carbs with something like a drill bit in the Mikuni manual.

    Idle needs to be set with the ski in the water. But first, I would adjust your idle air screws. There's a procedure for that I'm pretty sure i posted in this thread earlier. When that is done, then adjust the idle speed.

    Some carb spitting is normal. Excessive fuel spitting out the top is normally bad reeds. Your reeds look fine from what I can see.

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  3. #72

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    Ok. Here we go.

    I reinstalled the reed valves with new gaskets all the way around. I trimmed the spring in the 1st carb to match the pop off of the other two (roughly). I put new gaskets on the carbs. I replaced all tygon lines with reinforced fuel lines (except the return line). I synched the carbs, and dialed in the idle. All carb screws are as per the post from XLint89 a few back, with the exception of adding that 1/16 of a turn on the pto carb as someone else suggested. AND...

    I had it out, and at first it ran great. Was running just like it should. After maybe 15 minutes or so, it began to do that studdering thing and wouldn't get up to top speed again. So... One step forward and two back? In the video the first clip is it running as it should, then the second is as I'm headed back to the launch and it's sounding like a cylinder is missing again. I didn't pull the plugs at the ramp, but taking them out here (15 minute car ride home) they're dark but dry. I'm about ready to give up on this thing. Talk me off the cliff?


  4. #73

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    to clarify, the second clip doesn't show an attempt to get to full throttle, just the weird thing it does - it was maybe 3/8 throttle at the time.

  5. #74
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    If it ran fine for 15 mins, then something may be heat related, and failing when hot.

    Dry and black spark plugs is lean. Lack of fuel

    As you introduce more fuel, the black turns to a brown color. Which is what you want.

    Did you perform a plug chop test? Otherwise, you're just looking at the fuel supply of what the engine was doing right before you shut it off. Like idling back into the dock.

  6. #75

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    It may be something heat related yes. It sort of did this when I tried the break-in process for the new rings. The first few cycles it ran well (though I wasn't doing WOT) and then the later cycles it did the same studdering thing.

    Ring gaps wouldn't do that would they? Crank seal?

    Is that the next likely step? Adjust high speed screws or something?

  7. #76
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    High speed adjustment screws is really for fine tuning WOT. Your low and main jets take care of part throttle through 3/4 throttle. Depending on how lean your engine is, you might need larger jets.

    Take a look at your piston wash (hopefully you have enough time on the pistons for it to form) and see if they are all black on top. If so, you need more fuel. More than a simple screw adjustment would supply. (Meaning bigger jets)

    Get the ski warm and make it act up. Then do a spark test to see if you are losing spark when warm. Might be a magneto or coil failing.

    Or try slightly closing the choke and see if that makes it better. If closing the choke helps, you need more fuel.

    If closing the choke makes it worse, you might be giving it too much fuel.

    Once again, piston wash will help identify what might be going on.

  8. #77

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    Follow up questions:

    Increasing the size of the jets makes sense to me as the pistons were bored out 1mm over and the jets are still factory sized. I could see where the increased volume in the cylinder might need more fuel, but what I don't quite understand is why it would only affect one cylinder. Something to try though.

    Second question, would improperly gapped rings on that third cylinder potentially take that cylinder out as the piston expands with heat? I didn't have a feeler gauge at the time (I do now, though it's in mm) BUT the compression on that cylinder isn't super high, so I'm not sure I'd want to gap them any more? Thoughts?

  9. #78
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by nbarr7 View Post
    … the pistons were bored out 1mm over and the jets are still factory sized. …
    Oversized pistons have a minimal effect on combustion fuel:air ratio. Look for other factors.

    Correct piston sizing and ring gaps are important.
    As is having the rings correctly positioned on the groove locating pins.
    And having the rings right side up.

    Air leaks can come from many places (crankshaft end seals, reed seals, carb base gasket, even the pulse hose to the fuel pump) and these leaks can aggregate to cause lean fuel ratios.

    If the problem cause is electrical then messing with fueling will not resolve the problems. Adding partial choke when the problem is occurring can often provide useful diagnostic information.

    Have you ruled out exhaust gas leaks into the hull?

  10. #79
    BlueFishCrisis's Avatar
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    Quote Originally Posted by xlint89 View Post

    Get the ski warm and make it act up. Then do a spark test to see if you are losing spark when warm. Might be a magneto or coil failing.
    With it running well early, then lagging like in the video sounds like magneto to me. Almost feels/sounds like a limiter type action. Everything else, of course, is worth looking at.

  11. #80
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by nbarr7 View Post
    Follow up questions:

    Increasing the size of the jets makes sense to me as the pistons were bored out 1mm over and the jets are still factory sized. I could see where the increased volume in the cylinder might need more fuel, but what I don't quite understand is why it would only affect one cylinder. Something to try though.

    Second question, would improperly gapped rings on that third cylinder potentially take that cylinder out as the piston expands with heat? I didn't have a feeler gauge at the time (I do now, though it's in mm) BUT the compression on that cylinder isn't super high, so I'm not sure I'd want to gap them any more? Thoughts?
    Trick question. When these skis were manufactured, we had "good gas". Now we have this oxygenated, corn fed ethanol crap. So going strictly by the book may not generate the same results. It's not a bad idea to verify jetting regardless.

    Each cylinder needs to be treated as it's own separate engine. If you look at the service manual, some cylinders use different size jets in the same engine. The MAG cylinder gets the hottest, so it's fuel requirements might be different. The CEN cyl sees the least amount of load and might use less fuel than the other 2. And the PTO might have a slight air leak through the crank seal that requires more fuel than the other 2. You see what I'm getting at?

    Your piston rings have to do with cylinder compression and/or cylinder scuffing. If the gap is too big, you will have low compression and a poorly running cylinder. If the gap is too small, it would cause scuffing issues to the cylinder walls and possibly break.

    Now, with that being said, if it's the PTO cylinder with low compression, you might have an issue. The PTO cyl uses the positive (piston moving down) pressure and the negative (piston moving up) pressure to move the diaphragm inside the fuel pump back and forth rapidly to create vacuum to draw fuel in and pressure to force the fuel out to the carbs. With low compression comes a weak "signal" to operate the back and forth motion of the diaphragm. Thus resulting in lower fuel pressure, and that results in lower fuel supply. Lower fuel supply results in lower performance. You can see how this all relates.


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