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  1. #1

    Troubleshoot New Top End - running rough 787 Engine

    1999 SPX purchased as project ski - 787 engine

    1. The prior owner didn't know the problem, worked one weekend, the next weekend had problems. From several things he said, i suspect the engine may have been water flooded.
    2. Mag piston was 115. Mag piston showed severe smearing with frozen rings
    3. PTO was 150 just slightly scored, rings moved free.
    4. The shop diagnosis on the rebore was lack of oil

    What was done:

    1. New top end
    2. Rebuilt carbs
    3. the carb filters were not clean, but partially clogged, not filled with gunk though)
    4. New fuel lines (had the old grey ones - but didn't show much deterioration)
    5. Cleaned fuel selector
    6. New Oil injection lines (examined old ones after removal, appeared fine and full of oil)
    7. The oil tank appeared to have normal 2 cycle oil. Drained tank and oil lines. Bled oil lines. Primed oil pump and injection lines without plugs in the engine before starting. I feel confident the oil pump is working fine.

    Starting the engine

    1. Added oil to the fuel tank (64:1), with oil injection that should work out to 32:1.
    2. Started engine out of water for about 30 seconds, then off to the lake.
    3. Connected a timing light with an RPM gauge
    4. Left the boat on the trailer and started the engine.
    5. For minutes 0-10, ran fine but very smokey as expected
    6. For minutes 10-15, rough running like it was missing and occasional backfiring (that's what is sounds like to me)
    7. For minutes 15-20, smoothed back out was throttle responsive, but right at the end began the rough running)
    8. The water regulator was leaking quite a bit.
    Troubleshooting Ideas

    1. The only odd thing, was the gauge had issues reading the RPM's at times. My first thought was a connection or the timing light, I now wonder if there might be a spark issue?
    2. Water Regulator "clip spring" on the diaphragm was rusted away completely, going to swap out with the 99 xp.
    3. I have subsequently, started the boat on the trailer with water and experienced the same problems.
    4. I'm thinking new spark plugs, but these actually didn't look bad.
    5. I read about clipping the plug wires - can somebody elaborate?
    6. Toss your ideas this way, I would like to water test and ride this weekend.

  2. #2
    96XPSS's Avatar
    Join Date
    Feb 2006
    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
    To clip the wires remove the boots, and clip back 1/4" so the boot pin has clean wire to contact.

    It sounds like a busted inductive pickup to me. One side can crack, and give these symptoms.

    First check the rear black box fror dryness and corrosion on the ground lug. Although it's not my first choice, it's a possibility.

    Also reference this thread for pics of the pickup and location.

  3. #3
    Ok - update
    Doesn't want to accelerate beyond about 4500rpm - starts to run rough

    Did a little more testing to check the plug wires

    1. Alternately left each plug wire disconnected - started and ran
    2. Reversed plug wires and alternately left each plug wire disconnected - started and ran
    3. I'm leaning toward the opinion, this is the not the issue.
    RPM Inductive Meter

    1. Because of inconsistent readings, I tried it on my '98 GSXltd
    2. Identical results on the '98 GSX and the '99 SPX
      1. Reads on the PTO cylinder fine
      2. Never reads on the Mag cylinder
        1. Just realized as typing, i should swap the plug wires and see if it follows the wire
    3. FYI - have to halve the RPM reading to be accurate.

    Black Box

    1. Inspected - connections clean and good.
    2. At some point, somebody shorted one of the fuse wires to ground and rewired the 18awg wires.
    Other thoughts - all is welcome

    1. I'm leaning toward carburation?
      1. The carbs were rebuilt. LS - 1.5 turns out HS - no turns out

  4. #4
    maxadrenaline's Avatar
    Join Date
    Nov 2007
    Theodore, Alabama (Home) Shelbyville, KY (Work)
    I read in one of the threads here that the rectifier may cause a similar issue like the one you are having.

  5. #5
    Ok here is the latest update:

    Water tested last night in the rain and cold

    1. Now have a total of about 1.5 hours on the new top end - and started using the boat harder.
    2. The out of water problems are gone
    3. The boat runs rough at low speeds on only the low jets
      1. However - doesn't bog down, just runs rough above 2000rpm but before the high speed jets open
    4. With the High Speed jets open - boat runs great
    5. Max RPM is great, it gets up there and maintains without fall off
    6. Boat accelerates without hesitation

  6. #6
    dghal's Avatar
    Join Date
    Jul 2008
    League City Texas
    I would run some sea foam through it,, maybe there is a little something in the carb that it will clean up..

    good to hear you are running

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