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  1. #1

    SLT 780 Power surge at 4000 rpm

    I just finished rebuilding this ski. It runs great and very smooth above idle (still need to sync carbs, etc.) and a tad grouchy at idle.

    There's an unmistakeble spike in power right around 4000 RPM (around 20MPH) that I'm not sure exactly what's going on. The engine is very smooth so I'm assuming I'm getting all 3 cylinders firing and the CDI is at full advance at 3000 RPM (I read somewhere anyway).

    Is this the nature of the beast? Any idea what exactly is happening? Something with the reed valves?


    Also, I *think* the alignment is right but just wondered if there were any telltale sounds (growns, vibrations, etc.) when the alignment is off?


    Thanks
    Tom


  2. #2
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    At 4k, I think your getting in to the power band. "Getting on the pipe".
    Before I modded the crap outa my 780, thats where the acceleration really picked up on my ski. My tach is on the blink now, but I think its hitting a little sooner now, maybe around 3500? Hopefully I'll have a tach soon.
    Done any plug chops?

  3. #3

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    I have the same ski and i have a spike in power aswell....I was told its when i "get on the pipe" if you look in the general conversation area there is a topic that I started there about the throttle sweet spot where many have filled in what "getting on the pipe" means

  4. #4

    Surge is now a "hang up"

    The surge in power has now turned into getting "hung up" around 3600-4000rpm. A few seconds after giving it some juice, it'll take off like a rocket.

    The power surge seems more sudden than I'd expect from getting into a power curve -- my old Honda motorcycle had a very pronounced power curve and this is much more sudden than the motorcycle had.

    Here's the weird part: If, while it's hung up, I give it a tad of choke (like 1/8-1/4" pull out), it'll immediately take off.

    I was thinking pop-off valves not being in sync. Here's what I have:

    MAG: 20.0
    CEN: 20.0-20.5
    PTO: 21.0 - 21.5

    That seemed pretty close when I put the thing together but I'm wondering if a 1.5 psi spread is too great. How close is "close enough"?

    Since the situation is deteriorating and the choke making a difference seems to point to carbs, any ideas on what to look for? Does the choke on the Mikuni SBN carbs do something other than close off air? Any circuit changes?


    Thanks

  5. #5
    I *think* and dearly hope I found the problem. Tonight I pulled the carbs off (turned out not as bad as I thought it might be -- I put the engine in with them installed) and discovered that I somehow managed to put the center carb diaphragm bladder in backwards -- so the stub on the correct side wasn't pushing on the pop-off valve bladder.

    I put everything together tonight and hope to get her out on the water tomorrow!

  6. #6
    ph2ocraft's Avatar
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    Nice find, good luck.

  7. #7
    Ran it tonight and the problem is gone. Still feel a slight burp in power (jumps from 4000 to around 4600) but I think that's just the nature of the beasty given how the carbs transition to high volume fuel flows. Very manageable now and the ski runs like a top.

    Not sure how I managed to reverse the bladder on the center carb but sure am glad to have found it.

  8. #8
    Moderator beerdart's Avatar
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    Live strong and ride hard!!!!

  9. #9
    ph2ocraft's Avatar
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    4-4600 is just getting on the pipe, sounds normal.

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