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  1. #1
    Piston? WHAT Piston?!? JGringo's Avatar
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    What would cause EXTREME backfire in 780?

    I have my 780 Engine in a SL750 hull with an extended pump, aftermarket carbs, aftermarket waterbox, and shaved heads for premium gas compression. Sometimes when I hit the start button out of the water, its like someone just shot a shotgun right next to my head and litterally a burst of light comes out the back. It doesn't seem to hurt it because it starts up after it, and it never happens IN the water. But when I am just wrenching on it and diagnosing it, BAM! it blows my ear off and makes the neighbors call the police... Can it be fixed?


  2. #2
    sounds like unburnt fuel lighting in the exhaust ? or are you sure it's backfiring thru the carbs?

  3. #3
    Moderator RX951's Avatar
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    cracked reed(s) ?

    timing ?

    dribbling carbs ?

  4. #4
    Bing-A-Ding-Ding-Ding, Brrrrrap! Brrrrrrrrrap!!! Polaris_Nut#1's Avatar
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    My guess is reeds, and I wouldn't keep running it. I thought somebody on here said that can blow out a crank seal or maybe I'm dreaming? Guys?

  5. #5
    Piston? WHAT Piston?!? JGringo's Avatar
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    ok, well I checked all the reeds, They are flawless and practically new. The carbs I just re-routed for fuel and pulse lines after I cleaned them thoroughly with carb cleaner. The funny thing is that this only happens when its outta the water... Remember the engine was not designed for the application its in (SL750 hull). I think that the unburned fuel is sitting in the under the deck in the boxed in area around the pump. Literally, no damage to the internals like waterbox or tubing from the waterbox to the outside...

  6. #6
    A 780 won't have a clue it's in a 750 hull! somthings up?

  7. #7
    Piston? WHAT Piston?!? JGringo's Avatar
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    lol... well, I guess not! no... but seriously I found another god**** problem. I took the heads off, and the rear piston is burned, and concave right in the middle like a crater in the earth looks. PLUS, when I turn it over with my hand, the very front piston makes a high pitched squeeling sound but the cylinders are fine without massive scratching of any kind.

    I think someone hit a rock and it stopped the motor dead and bent all the rods. Then when the rods were bent he ran them so timing was off and thus provided the explosion and uncontrolled burning of the fuel on the rear piston.

    And because the rear piston is burned, I also think that is why I get a shotgun loud backfire out the exhaust pipe.

    But thats only my theory...

  8. #8

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    #1..You have an air leak that burnt a hold in your piston.
    #2..You lost fuel on the rear cylinder which burnt a hole.
    #3..Index is off from water intrusion/hydrolock or going from WOT with no engine load to instant load (in the air and landing with throttle at WOT)
    #4..Timing is way off for some reason.


    Backfiring can blow crank seals out, or break/ruin reed pedals.

  9. #9
    SeaLion's Avatar
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    Your timing is off or cdi is wigging out. Could explain the deto you are seeing on the piston too.

  10. #10
    Piston? WHAT Piston?!? JGringo's Avatar
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    Pretty sure that the WOT in the air to full load might be a little bit apart of it, but I am also sure the guy before me ran a rock through the impeller which could have bent the rod enough to put the timing off for deto... But I still have to diagnose the problem...

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